101 Best TypeScript Interview Questions and Answers for Experienced Professionals [2025]

TypeScript has become popular for many developers in the United States especially due to its strong typing and object-oriented features. As its adoption grows, companies increasingly seek experienced professionals with TypeScript skills. This tutorial will help developers prepare for TypeScript interviews by covering common questions and answers.

I selected 101 TypeScript interview questions and answers that will help experience developers to excel in their next interview. By preparing for these questions, developers can showcase their expertise and stand out from other candidates.

Table of Contents

1. What is TypeScript?

TypeScript is a programming language developed by Microsoft. It builds on JavaScript by adding optional static typing and other features.

TypeScript compiles to plain JavaScript, allowing it to run in any environment that supports JS. This includes browsers, Node.js, and more.

The language aims to make development easier, especially for large projects. It helps catch errors early through its type system and provides better tooling support.

TypeScript introduces concepts like interfaces, generics, and modules. These help developers write more structured and maintainable code.

Many popular frameworks and libraries have TypeScript support. This includes Angular, React, and Vue.js.

The language is open-source and constantly evolving. New features are added regularly based on developer feedback and needs.

TypeScript is widely used in enterprise settings. Its features make it well-suited for building complex applications.

Learning TypeScript can improve a developer’s skills and job prospects. It’s becoming increasingly common in job listings for front-end and full-stack roles.

typescript interview questions

Read How to Run TypeScript Files?

2. Key features of TypeScript

TypeScript adds static typing to JavaScript. This helps catch errors early in the development process. It makes code easier to understand and maintain.

TypeScript supports object-oriented programming concepts. These include classes, interfaces, and modules. They allow for better code organization and reusability.

The language offers enhanced IDE support. This includes auto-completion, navigation, and refactoring tools. These features boost developer productivity.

TypeScript is compatible with existing JavaScript code. Developers can gradually adopt TypeScript in their projects. This makes the transition smoother for teams.

It provides advanced type inference. The compiler can automatically deduce types in many cases. This reduces the need for explicit type annotations.

TypeScript supports ES6 and later JavaScript features. It allows use of modern syntax while targeting older JavaScript versions. This ensures broad browser compatibility.

The language includes generics. These enable creation of reusable components that work with different data types. Generics improve code flexibility and reduce duplication.

TypeScript offers union types and intersection types. These allow for more precise type definitions. They help model complex data structures accurately.

It supports optional parameters and default parameter values. This makes function declarations more flexible and expressive. It reduces the need for function overloading.

TypeScript includes enum support. Enums make it easier to work with sets of related constants. They improve code readability and maintainability.

3. TypeScript vs JavaScript

TypeScript and JavaScript are related programming languages, but they have some key differences. TypeScript is a superset of JavaScript, which means all valid JavaScript code is also valid TypeScript code.

TypeScript adds optional static typing to JavaScript. This allows developers to catch errors earlier in the development process. It also provides better tooling support and autocompletion in many code editors.

JavaScript is dynamically typed, meaning variables can hold any type of data. TypeScript introduces static typing, where developers can specify the types of variables, function parameters, and return values.

TypeScript requires compilation before it can run in a browser or Node.js environment. JavaScript, on the other hand, is interpreted and can run directly in these environments without compilation.

TypeScript offers additional features not found in JavaScript, such as interfaces, enums, and generics. These features help developers write more organized and maintainable code, especially for large projects.

JavaScript has wider browser support and can be used directly in web browsers. TypeScript needs to be transpiled to JavaScript before it can run in browsers.

TypeScript provides better code documentation through its type annotations. This can make it easier for developers to understand and work with large codebases.

Both languages have their strengths. JavaScript is more flexible and has a lower learning curve. TypeScript offers stronger type checking and additional features that can improve code quality and maintainability.

Check out Differences Between TypeScript and JavaScript

4. How to Install TypeScript

TypeScript installation is a straightforward process. Developers can use the Node Package Manager (npm) to set up TypeScript on their systems.

The first step is to ensure Node.js is installed on the computer. Node.js comes with npm, which is needed for the installation.

To install TypeScript globally, open a terminal or command prompt and type:

npm install -g typescript

This command downloads and installs the latest version of TypeScript. The ‘-g’ flag makes it available system-wide.

After installation, verify it by checking the version:

tsc --version

For project-specific installations, navigate to the project folder and run:

npm install typescript --save-dev

This adds TypeScript as a development dependency in the project’s package.json file.

Some IDEs and text editors, like Visual Studio Code, come with built-in TypeScript support. They may not require separate installation for basic use.

TypeScript can also be installed through other package managers like Yarn. The command for Yarn is:

yarn add typescript --dev

Once installed, developers can start using TypeScript in their projects. They can compile TypeScript files to JavaScript using the ‘tsc’ command.

Read Is TypeScript Frontend or Backend?

5. Setting up the TypeScript environment

Setting up a TypeScript environment is an important step for developers. The process is straightforward and can be done quickly.

First, install Node.js on your computer. This provides the npm package manager, which is needed to install TypeScript.

Next, open a command prompt or terminal. Use npm to install TypeScript globally by typing “npm install -g typescript”.

After installation, create a new folder for your project. Inside this folder, run “tsc –init” to generate a tsconfig.json file.

This file contains compiler options for your TypeScript project. You can adjust these settings as needed for your specific requirements.

Create a new .ts file in your project folder. Write some TypeScript code in this file to test your setup.

To compile your TypeScript code, run the “tsc” command in the terminal. This will create a JavaScript file from your TypeScript code.

You can also use the “tsc –watch” command to automatically compile your code whenever changes are made.

Many code editors, like Visual Studio Code, have built-in TypeScript support. These tools can make development easier and more efficient.

With these steps complete, your TypeScript environment is ready for use. You can now start writing and compiling TypeScript code for your projects.

Read How to Use Try Catch in TypeScript?

6. Type Inference in TypeScript

Type inference is a key feature in TypeScript. It allows the compiler to figure out types automatically. This happens when you don’t specify a type for a variable.

TypeScript looks at the value you assign to a variable. It then determines the most suitable type. This process makes coding faster and reduces errors.

For example, if you write ‘let x = 5’, TypeScript knows ‘x’ is a number. You don’t need to write ‘let x: number = 5’. The compiler does this work for you.

Type inference also works with complex types. If you create an object with specific properties, TypeScript infers its structure. This becomes the object’s type.

Functions benefit from type inference too. TypeScript can often deduce the return type of a function. It does this by looking at the function’s code and return statements.

Type inference helps catch errors early. If you try to use a variable in a way that doesn’t match its inferred type, TypeScript will alert you. This prevents many common coding mistakes.

While useful, type inference isn’t perfect. Sometimes you might need to specify types manually. This is especially true for complex scenarios or when you want to be more explicit.

Read How to Handle Catch Error Types in TypeScript?

7. Type Annotations

Type annotations are a key feature of TypeScript. They let developers specify the type of a variable, function parameter, or return value.

Using type annotations helps catch errors early in the development process. It also makes code more readable and self-documenting.

The syntax for type annotations is simple. After a variable name or function parameter, add a colon followed by the type.

For example, to declare a variable as a number:

let age: number = 30;

Function parameters can also have type annotations:

function greet(name: string) {
  console.log(`Hello, ${name}!`);
}

Type annotations are optional in TypeScript. The compiler can often infer types automatically. However, adding them explicitly can improve code clarity and prevent mistakes.

TypeScript supports many built-in types. These include number, string, boolean, and array. It also allows custom types and interfaces for more complex data structures.

Using type annotations helps create more robust and maintainable code. It’s a valuable skill for TypeScript developers to master.

Read Difference Between == and === in TypeScript

8. Interfaces in TypeScript

Interfaces in TypeScript define the structure of objects. They specify what properties and methods an object should have.

Interfaces are useful for type-checking. They help catch errors before runtime if an object doesn’t match the expected shape.

To create an interface, use the “interface” keyword followed by the interface name. Then list the properties and their types inside curly braces.

Interfaces can include optional properties. Add a question mark after the property name to make it optional.

You can also define method signatures in interfaces. This ensures objects implement specific functions with the correct parameters and return types.

TypeScript allows extending interfaces. This lets you create new interfaces based on existing ones, adding extra properties or methods.

Interfaces support readonly properties. Use the “readonly” keyword before a property to prevent it from being changed after initial assignment.

Classes can implement interfaces using the “implements” keyword. This ensures the class includes all the properties and methods defined in the interface.

Interfaces can describe function types too. This is helpful when working with callbacks or defining the shape of function parameters.

TypeScript’s structural typing system means objects don’t need to explicitly declare they implement an interface. They just need to have the right shape.

Read How to Set Default Values in TypeScript Interfaces?

9. Classes in TypeScript

Classes in TypeScript provide a way to create reusable components. They are a core part of object-oriented programming in TypeScript. Classes can have properties and methods, which define their structure and behavior.

To create a class, use the ‘class’ keyword followed by the class name. Inside the class, you can define properties and methods. Properties are variables that hold data, while methods are functions that perform actions.

TypeScript classes support inheritance. This means a class can extend another class, inheriting its properties and methods. Use the ‘extends’ keyword to create a child class that inherits from a parent class.

Classes can also have constructors. These are special methods that run when a new instance of the class is created. Constructors set up the initial state of the object.

Access modifiers like ‘public’, ‘private’, and ‘protected’ control the visibility of class members. ‘Public’ members can be accessed from anywhere, ‘private’ members are only accessible within the class, and ‘protected’ members are accessible within the class and its subclasses.

TypeScript also supports abstract classes. These are base classes that can’t be instantiated directly. They serve as templates for other classes to inherit from.

Classes can implement interfaces in TypeScript. This ensures that a class adheres to a specific contract, implementing all the properties and methods defined in the interface.

Read Differences Between Type and Interface in TypeScript

10. Public, Private, and Protected Modifiers in TypeScript

TypeScript offers access modifiers to control the visibility of class members. These modifiers help developers create more secure and organized code.

Public is the default modifier. It allows access to class members from anywhere, inside or outside the class. Public members can be freely used by other parts of the program.

Private restricts access to class members. Only code within the same class can use private members. This helps hide internal details and prevents unwanted changes from outside the class.

Protected is similar to private, but allows access in child classes too. This is useful for creating base classes with some shared functionality that derived classes can use and extend.

Here’s a simple example:

class Person {
  public name: string;
  private age: number;
  protected id: string;

  constructor(name: string, age: number, id: string) {
    this.name = name;
    this.age = age;
    this.id = id;
  }
}

In this code, ‘name’ can be accessed from anywhere. ‘age’ can only be used inside the Person class. ‘id’ can be used in Person and any classes that inherit from it.

These modifiers help create clear boundaries in code. They support the principle of encapsulation, a key concept in object-oriented programming.

Read How to Add Functions to Interfaces in TypeScript?

11. Readonly modifier in TypeScript

The readonly modifier in TypeScript is a powerful feature that helps developers create immutable properties. It prevents the modification of a property after its initial assignment.

When applied to properties, readonly ensures they can only be set during initialization or in the constructor of a class. This adds an extra layer of safety to code by preventing accidental changes.

Readonly can be used with various types, including primitives, objects, and arrays. For primitive types, it’s straightforward – the value cannot be changed once set.

With objects and arrays, readonly prevents reassignment of the reference. However, the internal properties or elements can still be modified unless additional measures are taken.

TypeScript’s compiler enforces the readonly rule. It will raise an error if there’s an attempt to modify a readonly property outside of its allowed contexts.

Using readonly is beneficial for creating more predictable and maintainable code. It clearly communicates intent and helps prevent bugs caused by unintended modifications.

Readonly properties are particularly useful in interfaces and type definitions. They allow developers to define contracts that guarantee certain properties won’t change throughout the lifecycle of an object.

In practice, readonly is often used for configuration objects, constants, and properties that should remain stable after initialization. This makes code easier to reason about and reduces the potential for errors.

typescript interview questions and answers for experienced

12. Enforcing Interfaces

Interfaces in TypeScript help define the structure of objects. They act as contracts that specify what properties and methods an object should have.

To enforce an interface, developers use the “implements” keyword when creating a class. This ensures the class includes all the required properties and methods defined in the interface.

Here’s a simple example:

interface Person {
  name: string;
  age: number;
  greet(): void;
}

class Employee implements Person {
  name: string;
  age: number;

  constructor(name: string, age: number) {
    this.name = name;
    this.age = age;
  }

  greet() {
    console.log(`Hello, I'm ${this.name}`);
  }
}

In this case, the Employee class must include the name and age properties, as well as the greet method. If any of these are missing, TypeScript will show an error.

Interfaces can also be used with object literals. TypeScript checks if the object has all the required properties and methods.

let worker: Person = {
  name: "John",
  age: 30,
  greet: () => console.log("Hi there!")
};

Enforcing interfaces helps catch errors early in the development process. It ensures that objects have the expected structure and behavior.

TypeScript’s type checking system uses interfaces to provide better code completion and refactoring tools. This makes development faster and more efficient.

Read TypeScript Generic Anonymous Functions

13. Optional Properties in TypeScript Interfaces

Optional properties in TypeScript allow developers to create more flexible object types. These properties may or may not exist in an object, making them useful for scenarios where certain fields are not always required.

To define an optional property, developers use a question mark (?) after the property name. This tells TypeScript that the property is optional and can be omitted without causing errors.

Here’s a simple example of an object type with optional properties:

interface Person {
  name: string;
  age?: number;
  email?: string;
}

In this case, ‘name’ is required, while ‘age’ and ’email’ are optional. Objects of type Person can include or exclude these optional properties.

Optional properties are especially helpful when working with APIs or form data. They allow for partial updates or incomplete information without compromising type safety.

When accessing optional properties, it’s important to check if they exist first. This prevents errors that might occur if the property is undefined. TypeScript provides ways to handle this, such as the optional chaining operator (?.).

Optional properties enhance TypeScript’s type system by adding flexibility while maintaining strong typing. They help create more adaptable and reusable interfaces, improving overall code quality and developer experience.

Read How to Use TypeScript Optional Function Parameters?

14. Enum types in TypeScript

Enum types in TypeScript are a way to define a set of named constants. They make code more readable and help prevent errors by limiting the possible values a variable can take.

To create an enum, use the enum keyword followed by the enum name and a list of members. For example:

enum Color {
  Red,
  Green,
  Blue
}

By default, enum members are assigned numeric values starting from 0. You can also set custom values:

enum Direction {
  Up = 1,
  Down = 2,
  Left = 3,
  Right = 4
}

Enums can have string values too:

enum Status {
  Active = "ACTIVE",
  Inactive = "INACTIVE",
  Pending = "PENDING"
}

To use an enum, reference it by its name and member:

let myColor: Color = Color.Green;
let myStatus: Status = Status.Active;

Enums are useful for representing a fixed set of options, such as days of the week, card suits, or menu choices. They improve code clarity and help catch errors at compile-time.

TypeScript also supports const enums, which are removed during compilation for better performance. Declare them using the const keyword before enum:

const enum Planet {
  Earth,
  Mars,
  Jupiter
}

Enums in TypeScript provide a convenient way to work with sets of related constants, enhancing code readability and type safety.

Read How to Specify Return Type in TypeScript Arrow Functions?

15. TypeScript vs ES6

TypeScript and ES6 (ECMAScript 2015) are both important in modern web development. TypeScript builds on ES6, adding extra features.

ES6 brought new syntax and capabilities to JavaScript. It introduced let and const for variable declaration, arrow functions, and classes.

TypeScript includes all ES6 features and adds static typing. This means developers can specify variable types, making code more predictable and easier to debug.

Another key difference is that TypeScript needs to be compiled into JavaScript. ES6 code can run directly in modern browsers without compilation.

TypeScript offers interfaces and generics, which ES6 doesn’t have. These features help create more robust and reusable code.

ES6 is pure JavaScript, so it has a gentler learning curve for JavaScript developers. TypeScript requires learning new concepts and syntax.

TypeScript provides better tooling support. Its static typing enables more accurate code completion and refactoring in many development environments.

Both TypeScript and ES6 aim to improve JavaScript development. The choice between them depends on project needs and team preferences.

Read How to Use Default Parameter Values in TypeScript?

16. TypeScript’s tsc command

The tsc command is a key tool for TypeScript developers. It stands for TypeScript Compiler and is used to convert TypeScript code into JavaScript.

Developers run tsc in the command line to compile their TypeScript files. This process checks for errors and generates JavaScript output that can run in browsers or Node.js environments.

The basic usage is simple. Typing “tsc filename.ts” will compile a single TypeScript file. For larger projects, developers often use a tsconfig.json file to set compiler options.

Tsc offers many useful flags. The –watch flag is popular, as it automatically recompiles files when changes are detected. This saves time during development.

Another helpful flag is –outDir, which specifies where compiled JavaScript files should be placed. This keeps TypeScript source files separate from the generated JavaScript.

The –target flag allows developers to choose which version of JavaScript to compile to. This is important for maintaining compatibility with different environments.

Tsc also provides type-checking without compilation using the –noEmit flag. This is useful for quickly finding errors without generating JavaScript files.

Understanding tsc and its options is essential for effective TypeScript development. It forms the foundation of the TypeScript workflow and enables developers to leverage the language’s full potential.

17. Compiling options in TypeScript

TypeScript offers various compiling options to customize the output of the transpilation process. These options can be set in the tsconfig.json file or passed as command-line arguments.

One important option is “target”. It specifies the ECMAScript version for the compiled JavaScript code. Common values include “es5”, “es6”, and “esnext”.

The “module” option determines the module system for the output code. Choices include “commonjs”, “amd”, “umd”, and “es6”.

“strict” is a flag that enables all strict type-checking options. It helps catch more errors during development.

The “outDir” option sets the output directory for compiled files. This keeps the project organized by separating source and compiled code.

“sourceMap” generates corresponding .map files. These files help with debugging by mapping compiled code back to the original TypeScript source.

“noImplicitAny” prevents variables from being implicitly typed as “any”. This ensures better type safety throughout the codebase.

The “removeComments” option strips comments from the output JavaScript files. This can reduce file size in production builds.

“declaration” generates .d.ts files alongside the JavaScript output. These declaration files are useful for type checking in other projects.

Developers can use these options to tailor the TypeScript compilation process to their project’s specific needs.

Read How to Return Multiple Values in TypeScript?

18. Union Types

Union types in TypeScript let developers combine multiple types into one. This feature is useful when a value can be of more than one type.

To create a union type, use the pipe symbol (|) between the types. For example, a variable that can be either a string or a number would be written as:

let value: string | number;

Union types are helpful in functions that can accept different types of arguments. They allow for more flexible code without sacrificing type safety.

TypeScript will only allow operations that are valid for all types in the union. To use type-specific operations, developers need to narrow down the type first.

Type narrowing can be done using type guards or type assertions. This helps TypeScript understand which specific type is being used at a given point in the code.

Union types can include any number of types, including custom types and interfaces. This makes them very versatile for handling complex data structures.

When working with union types, it’s important to handle all possible cases to avoid runtime errors. TypeScript’s type checking helps catch potential issues during development.

Read How to Get the Return Type of a Function in TypeScript?

19. Intersection Types

Intersection types in TypeScript allow developers to combine multiple types into one. This feature is useful when you need an object to fulfill the requirements of two or more types at once.

To create an intersection type, you use the ‘&’ symbol between the types you want to combine. For example, you can merge two interfaces into a single type.

Let’s say you have a ‘Person’ interface with name and age properties, and a ‘Worker’ interface with job and salary properties. You can create a ‘WorkingPerson’ type that includes all these properties.

Here’s how you would define it:

type WorkingPerson = Person & Worker;

This new type will have all the properties from both ‘Person’ and ‘Worker’. It’s a powerful way to create complex types without duplicating code.

Intersection types are often used with object types, but they can also be used with other types. They’re particularly helpful when working with mixins or when you want to combine capabilities from multiple sources.

Remember that when using intersection types, an object must satisfy all the requirements of the combined types. This ensures type safety and helps catch errors early in development.

20. Type Guards

Type guards are a feature in TypeScript that help narrow down the type of a variable within a conditional block. They allow developers to check the type of a value at runtime and act accordingly.

Type guards use special checks to determine if a variable belongs to a specific type. This is useful when working with union types or when the exact type of a value is unknown.

There are several ways to create type guards in TypeScript. The most common method is using the typeof operator for primitive types like string, number, or boolean.

Another approach is the instanceof operator, which checks if an object is an instance of a particular class or constructor function. This is helpful when dealing with custom classes or built-in objects.

TypeScript also provides a way to create custom type guards using user-defined type predicates. These are functions that return a boolean value and help TypeScript understand the type of a variable in a specific context.

Type guards improve code safety and readability by allowing developers to write more specific and type-safe code. They reduce the need for type assertions and help catch potential errors early in development.

When used effectively, type guards can lead to cleaner and more maintainable code. They enable developers to write more precise conditional logic based on the actual types of variables at runtime.

Read How to Use Default Function Parameters in TypeScript?

21. Type Aliases

Type aliases in TypeScript let developers create custom names for types. They make code more readable and help avoid repeating complex type definitions.

To create a type alias, use the ‘type’ keyword followed by a name and an equals sign. Then define the type structure.

Type aliases can represent simple types, union types, or complex object shapes. They’re useful for giving clear names to function parameter types or return values.

Unlike interfaces, type aliases can’t be extended or implemented. They’re best for creating union types or tuple types.

Type aliases are helpful when working with complex types that are used in multiple places. They reduce code duplication and improve maintainability.

Developers often use type aliases to create more descriptive names for existing types. This makes the code’s intent clearer to other team members.

Type aliases are resolved statically at compile time. This means they don’t add any runtime overhead to the JavaScript output.

When deciding between interfaces and type aliases, consider the specific needs of your project. Both have their strengths in different situations.

Read How to Pass a Function as a Parameter in TypeScript?

22. Generics in TypeScript

Generics in TypeScript allow developers to create reusable components that work with different data types. They provide flexibility and type safety when writing functions, classes, or interfaces.

The syntax for generics uses angle brackets with a type parameter, often denoted as . This T can represent any type, which is determined when the code is used.

A common example of generics is the identity function. It returns the same value that was passed in, regardless of its type.

function identity<T>(arg: T): T {
  return arg;
}

Generics can also be used with interfaces and classes. This enables the creation of flexible, reusable structures that maintain type safety.

Arrays are a built-in generic type in TypeScript. The Array notation specifies an array that contains elements of type T.

Generics support constraints, which limit the types that can be used. This is done with the extends keyword. For example, restricts T to number types.

Multiple type parameters are possible in generics. This allows for more complex relationships between types in a single component.

Generics help catch errors at compile-time rather than runtime. This leads to more robust and maintainable code, especially in large projects.

Read How to Use the Double Question Mark (??) Operator in TypeScript?

23. Modules in TypeScript

Modules in TypeScript help organize code into separate files. They allow developers to split large programs into smaller, more manageable pieces.

TypeScript modules use the import and export keywords. These keywords control what parts of a file are visible to other files.

The export keyword makes functions, classes, or variables available for use in other files. Developers can export single items or multiple items at once.

To use exported items in another file, the import keyword is used. This brings in the specific parts needed from other modules.

Modules in TypeScript support different import styles. These include default imports, named imports, and namespace imports.

Default imports are used when a module exports a single main item. Named imports allow bringing in specific parts of a module. Namespace imports group all exports under a single name.

TypeScript modules help prevent naming conflicts. They create a local scope for variables and functions. This means names in one module won’t clash with names in another.

Modules also enable better code reuse. Developers can write a piece of code once and use it in many places. This makes programs easier to maintain and update.

When working with modules, TypeScript checks types across file boundaries. This helps catch errors early in the development process.

Read How to Use Optional Parameters in TypeScript Interfaces?

24. Namespaces in TypeScript

Namespaces in TypeScript provide a way to organize code and avoid naming conflicts. They group related functionality into a single container, making it easier to manage large codebases.

TypeScript namespaces are declared using the ‘namespace’ keyword. They can contain classes, interfaces, functions, and variables. This helps keep code modular and prevents global scope pollution.

Namespaces can be nested within each other, allowing for more complex organizational structures. This is useful for creating hierarchies of related functionality.

To use elements from a namespace outside of it, developers must export them. The ‘export’ keyword makes items accessible to other parts of the code.

When working with multiple files, namespaces can be split across different files using reference tags. This allows for better code organization and separation of concerns.

Namespaces are compiled into JavaScript using the module pattern. This ensures compatibility with various JavaScript environments.

While namespaces are still supported in TypeScript, modules are now the preferred method for organizing code. Modules offer better compatibility with modern JavaScript practices and tools.

Read Define and Use TypeScript Interface Array of Objects

25. TypeScript Decorators

TypeScript decorators are special kinds of declarations that can be attached to classes, methods, properties, or parameters. They provide a way to add metadata or modify the behavior of code elements.

Decorators use the “@” symbol followed by the decorator name. They are applied right before the declaration they’re decorating. This feature allows developers to enhance or change how parts of their code work without changing the core logic.

Class decorators can modify or replace class definitions. Method decorators can observe, change, or replace method definitions. Property decorators can be used to observe, modify, or replace property definitions.

Decorators are still an experimental feature in TypeScript. To use them, developers need to enable the “experimentalDecorators” compiler option in their tsconfig.json file.

A simple example of a class decorator might look like this:

function sealed(constructor: Function) {
  Object.seal(constructor);
  Object.seal(constructor.prototype);
}

@sealed
class Greeter {
  greeting: string;
  constructor(message: string) {
    this.greeting = message;
  }
  greet() {
    return "Hello, " + this.greeting;
  }
}

In this example, the @sealed decorator prevents the Greeter class from being modified after its definition. This can be useful for ensuring code stability and preventing unintended changes.

Read Convert JSON to TypeScript Interface

26. Asynchronous programming with TypeScript

TypeScript supports asynchronous programming through various mechanisms. Promises are a key feature for handling asynchronous operations. They represent a value that may not be available immediately but will be resolved at some point in the future.

Async/await syntax builds on top of Promises, making asynchronous code easier to write and read. The ‘async’ keyword is used to define a function that returns a Promise, while ‘await’ is used inside async functions to wait for Promise resolution.

TypeScript also provides type annotations for Promises. This allows developers to specify the type of value a Promise will resolve to, enhancing type safety in asynchronous code.

Callbacks are another method for handling asynchronous operations in TypeScript. While less modern than Promises, they are still used in some scenarios.

Error handling in asynchronous TypeScript code can be done using try/catch blocks with async/await, or .catch() methods with Promises.

TypeScript’s type system helps catch potential errors in asynchronous code during development. It can identify issues like incorrect Promise chain usage or mismatched types in async functions.

Generators and iterators can also be used for more complex asynchronous patterns in TypeScript. These allow for pausable functions and custom iteration behavior.

TypeScript’s asynchronous features integrate well with APIs that return Promises, such as the Fetch API for making HTTP requests.

Read Set Default Values for TypeScript Types

27. Promises in TypeScript

Promises in TypeScript offer a way to handle asynchronous operations. They represent a value that may not be available right away but will be resolved at some point in the future.

TypeScript provides full support for Promises, including type checking. Developers can specify the type of value a Promise will resolve to, enhancing code clarity and reducing errors.

Creating a Promise in TypeScript is straightforward. The Promise constructor takes a function with two parameters: resolve and reject. These functions are used to indicate the success or failure of the asynchronous operation.

Promises have three states: pending, fulfilled, or rejected. A pending Promise can either be fulfilled with a value or rejected with a reason. Once settled, a Promise stays in that state.

TypeScript allows chaining Promise methods like .then() and .catch(). This enables clean handling of asynchronous code without nested callbacks. The .then() method is used for successful outcomes, while .catch() handles errors.

Async/await syntax, built on top of Promises, is also supported in TypeScript. This feature makes asynchronous code look and behave more like synchronous code, improving readability.

Promise.all() and Promise.race() are utility methods for working with multiple Promises. Promise.all() waits for all Promises to resolve, while Promise.race() resolves as soon as any Promise in the array settles.

TypeScript’s type system helps catch common mistakes when working with Promises. It can prevent errors like forgetting to handle rejections or misusing Promise methods.

Read Check the Type of a Variable in TypeScript

28. Async/Await in TypeScript

Async/Await is a feature in TypeScript that makes working with asynchronous code easier. It allows developers to write asynchronous code that looks and behaves like synchronous code.

The ‘async’ keyword is used to define a function that returns a Promise. Inside an async function, the ‘await’ keyword can be used to pause execution until a Promise is resolved.

This approach helps avoid callback hell and makes code more readable. It’s especially useful when dealing with multiple asynchronous operations that depend on each other.

Error handling in async/await is done using try-catch blocks. This is more intuitive than using .catch() with Promises.

TypeScript adds type checking to async/await. It ensures that the awaited value is actually a Promise, catching potential errors at compile-time.

Async functions can be used with arrow function syntax as well. This is common in React components and other scenarios where concise function definitions are preferred.

When using async/await, it’s important to remember that the function will always return a Promise. Even if you return a non-Promise value, TypeScript will wrap it in a Promise.

Async/await can be used with Promise.all() to handle multiple asynchronous operations concurrently. This is useful for improving performance when operations don’t depend on each other.

TypeScript’s type inference works well with async/await. It can often correctly infer the return type of an async function based on the awaited Promises.

29. TypeScript with React

TypeScript and React work well together. TypeScript adds static typing to JavaScript, which helps catch errors early in development.

When using TypeScript with React, developers can define prop types and state types for components. This improves code quality and makes it easier to understand component interfaces.

TypeScript allows for better autocompletion and refactoring in React projects. IDEs can provide more accurate suggestions based on defined types.

React components can be written as TypeScript classes or function components. Both approaches benefit from TypeScript’s type checking.

Hooks in React also work smoothly with TypeScript. Developers can specify types for useState and useEffect, making their usage clearer.

TypeScript helps prevent common React errors like passing the wrong prop types. It catches these issues at compile-time rather than runtime.

For larger React applications, TypeScript’s ability to scale and maintain code becomes even more valuable. It helps teams work together more efficiently.

Many popular React libraries now come with TypeScript definitions. This makes it easier to use third-party components in a type-safe manner.

While there’s a learning curve, using TypeScript with React often leads to more robust and maintainable code in the long run.

30. TypeScript with Node.js

TypeScript works well with Node.js, allowing developers to build server-side applications with enhanced type safety. To use TypeScript in a Node.js project, developers need to install the TypeScript compiler and any necessary type definitions.

The setup process involves creating a tsconfig.json file to configure TypeScript compilation options. This file specifies settings like the output directory, target JavaScript version, and module system.

Node.js developers can leverage TypeScript’s static typing to catch errors early in the development process. This helps improve code quality and reduces runtime errors in production environments.

TypeScript also provides better tooling support for Node.js projects. IDEs can offer more accurate code completion and refactoring tools when working with TypeScript files.

Using TypeScript with Node.js requires an extra compilation step before running the code. Developers can set up build scripts to automate this process, making it seamless to work with TypeScript in Node.js projects.

TypeScript’s type system allows for easier maintenance of large-scale Node.js applications. It helps teams collaborate more effectively by providing clear interfaces and type definitions for modules and functions.

Popular Node.js frameworks like Express can be used with TypeScript, providing type definitions for improved development experience. This combination allows for building robust and scalable server-side applications.

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31. TypeScript Configuration File

The TypeScript configuration file is a JSON file named tsconfig.json. It’s placed in the root of a TypeScript project. This file tells the TypeScript compiler how to handle the project’s files.

The tsconfig.json file sets various options for compiling TypeScript code. It can specify which files to include or exclude from compilation. It also sets the output directory for compiled JavaScript files.

Some key options in tsconfig.json include “target” for setting the ECMAScript version, and “module” for choosing the module system. The “strict” option enables stricter type-checking rules.

Developers can customize the compiler’s behavior with options like “noImplicitAny” and “strictNullChecks”. These help catch potential errors early in development.

The “lib” option allows specifying which built-in APIs should be available. This is useful for targeting specific JavaScript environments.

Using a tsconfig.json file makes it easier to maintain consistent compiler settings across a project. It also simplifies the build process, especially for larger codebases.

Most IDEs and code editors recognize tsconfig.json. They use it to provide better TypeScript support and IntelliSense features.

32. Handling TypeScript errors

TypeScript helps catch errors early in the development process. It uses static type checking to find issues before code runs. This can save time and prevent bugs in production.

Developers can use the TypeScript compiler to spot errors. The compiler shows warnings and errors in the code. It points out type mismatches, undefined variables, and other common mistakes.

IDE integration is key for handling TypeScript errors. Many code editors provide real-time error highlighting. This helps programmers fix issues as they type.

TypeScript also offers ways to suppress errors when needed. The “any” type can be used to bypass type checking. Developers can use type assertions to tell the compiler about a specific type.

Error handling in TypeScript includes try-catch blocks. These blocks help manage runtime errors. They allow the program to continue running even if part of it fails.

The TypeScript config file (tsconfig.json) can be adjusted to control error reporting. Developers can set strict mode for more thorough checks. They can also disable certain error checks if needed.

TypeScript introduces the concept of “strictNullChecks”. This feature helps prevent null and undefined errors. It forces developers to handle these cases explicitly.

Type guards are useful for narrowing down types and avoiding errors. They allow TypeScript to infer more specific types within conditional blocks. This leads to safer code and fewer runtime errors.

33. TypeScript Mixins

TypeScript mixins are a way to add functionality to classes without using inheritance. They allow developers to combine multiple classes into one, creating a new class with the features of all the combined classes.

Mixins are useful when you want to add behaviors to a class without changing its core structure. This technique helps avoid the limitations of single inheritance in TypeScript.

To create a mixin, you typically define a function that takes a base class as an argument and returns a new class. This new class extends the base class and adds new properties or methods.

Here’s a simple example of a mixin:

function Timestamped<T extends new (...args: any[]) => {}>(Base: T) {
  return class extends Base {
    timestamp = Date.now();
  };
}

You can then use this mixin to add a timestamp to any class:

class User {
  name: string;
  constructor(name: string) {
    this.name = name;
  }
}

const TimestampedUser = Timestamped(User);
const user = new TimestampedUser("John");
console.log(user.timestamp);

Mixins are great for code reuse and can help keep your codebase more modular and flexible. They’re often used in scenarios where you need to add similar functionality to multiple unrelated classes.

34. Discriminated Unions

Discriminated unions are a powerful feature in TypeScript. They allow developers to work with several related types in a type-safe way.

A discriminated union uses a common property to distinguish between different types. This property is often called the “discriminant” or “tag.”

To create a discriminated union, define an interface for each type in the union. Each interface should have a common property with a unique literal type.

Here’s a simple example:

interface Square {
  kind: "square";
  size: number;
}

interface Circle {
  kind: "circle";
  radius: number;
}

type Shape = Square | Circle;

In this case, “kind” is the discriminant property. It helps TypeScript determine which type is being used.

When working with discriminated unions, TypeScript can infer the correct type based on the discriminant. This makes it easy to write type-safe code:

function area(shape: Shape): number {
  if (shape.kind === "square") {
    return shape.size * shape.size;
  } else {
    return Math.PI * shape.radius * shape.radius;
  }
}

Discriminated unions are useful for modeling complex data structures. They help catch errors at compile-time and make code more maintainable.

35. TypeScript Utility Types

TypeScript utility types are built-in types that help developers manipulate and transform existing types. They provide powerful tools for working with type definitions and enhancing code flexibility.

One common utility type is Partial. It creates a new type with all properties of T set to optional. This is useful when updating objects where only some fields may change.

Another helpful utility is Pick<T, K>. It constructs a type by picking specific properties K from type T. This allows for creating new types with only selected fields from an existing type.

Readonly is a utility that makes all properties of T read-only. It’s valuable for ensuring immutability in certain parts of the codebase.

Exclude<T, U> creates a type by removing all properties from T that are assignable to U. This is handy for filtering out unwanted types from a union.

Record<K, T> is used to create an object type with keys of type K and values of type T. It’s great for defining dictionaries or maps with specific key and value types.

NonNullable removes null and undefined from type T. This helps create more precise types when working with potentially nullable values.

ReturnType extracts the return type of a function type T. It’s useful for capturing and reusing the output type of a function.

These utility types simplify complex type manipulations and promote code reusability. They help developers write more robust and type-safe TypeScript code.

36. Record types

Record types in TypeScript are useful for creating objects with keys of specific types and values of another type. They provide a way to define flexible object structures.

The syntax for a Record type is Record<Keys, Type>. Keys represent the allowed property names, while Type specifies the value type for those properties.

Here’s an example of a Record type:

type Fruit = Record<string, number>;

const fruitInventory: Fruit = {
  apples: 10,
  bananas: 15,
  oranges: 20
};

In this case, Fruit is a Record type where keys are strings and values are numbers. This allows for any string key with a number value.

Record types are especially handy when working with objects that have a known shape but varying property names. They ensure type safety for both keys and values.

Another common use case is creating dictionaries or lookup tables:

type UserRoles = Record<string, 'admin' | 'user' | 'guest'>;

const roles: UserRoles = {
  alice: 'admin',
  bob: 'user',
  charlie: 'guest'
};

This example defines UserRoles as a Record type with string keys and specific role values. It enforces that all entries in the roles object follow this structure.

37. Mapped Types

Mapped types in TypeScript allow developers to create new types based on existing ones. They provide a way to transform properties of an existing type into a new type.

Mapped types use a syntax similar to index signatures. They iterate over a union of property keys to create a new type.

A common use case for mapped types is to make all properties of an object optional. This can be done using the Partial utility type.

Another example is making all properties of an object readonly. The Readonly utility type accomplishes this task.

Developers can also create custom mapped types. For instance, they might want to change the type of every property in an object to a string.

Mapped types are powerful for working with dynamic data structures. They help maintain type safety while allowing flexibility in object shapes.

When using mapped types, developers can modify property types, add modifiers, or even filter out certain properties. This makes them versatile for various type transformations.

TypeScript’s built-in utility types, like Pick and Omit, are examples of mapped types. These utilities help create new types by selecting or excluding specific properties from existing types.

38. Using keyof in TypeScript

The keyof operator in TypeScript is a powerful tool for working with object types. It allows developers to create new types based on the keys of existing object types.

When used on an object type, keyof returns a union of the object’s property names as string literal types. This can be very useful for creating more flexible and type-safe code.

For example, if you have an interface Person with properties name and age, using keyof Person would give you the type “name” | “age”. This new type can be used in function parameters or as constraints for generic types.

One common use case for keyof is creating functions that can access any property of an object safely. By using keyof as a generic constraint, you can ensure that only valid property names are passed to the function.

Another practical application is creating mapped types. These allow you to transform an existing type by applying a specific operation to each of its properties.

The keyof operator can also be combined with typeof to get the keys of a specific object instance rather than a type. This pattern is often used when working with constants or configuration objects.

Using keyof helps catch errors at compile-time and improves code reliability. It enables developers to write more generic and reusable code while maintaining strong type safety.

39. Conditional Types

Conditional types in TypeScript allow developers to create flexible and dynamic type definitions. They enable the selection of types based on certain conditions.

Conditional types use a syntax similar to ternary operators in JavaScript. They take the form T extends U ? X : Y, where T is checked against U.

If T is assignable to U, the type X is chosen. Otherwise, the type Y is selected. This feature is useful for creating more precise and adaptable type definitions.

Developers can use conditional types to build complex type relationships. These relationships can depend on the properties or structure of other types.

Conditional types often work alongside other TypeScript features like mapped types and union types. This combination allows for powerful type manipulation and inference.

One common use case for conditional types is in generic functions. They help create more specific return types based on the input parameters.

TypeScript’s type inference system can leverage conditional types to provide better type information. This leads to improved code completion and error detection in IDEs.

Advanced TypeScript developers frequently use conditional types to create utility types. These utility types can transform or extract information from existing types.

Learning to use conditional types effectively can greatly enhance a developer’s ability to write type-safe and flexible TypeScript code.

40. Template Literal Types

Template literal types in TypeScript allow developers to create complex string types based on existing types. They use backticks and work similarly to template literals in JavaScript.

These types can combine string literals with other types to form new string types. This feature is useful for creating more specific and flexible type definitions.

Developers can use template literal types to represent patterns in strings. For example, they can define types for file paths, URL structures, or specific string formats.

Template literal types support string interpolation with placeholders. These placeholders can be filled with other types, including unions, to create dynamic string types.

This feature enables the creation of more precise types for string-based operations. It helps catch errors related to string formatting at compile-time rather than runtime.

Template literal types can also be used with mapped types and conditional types. This combination allows for powerful type transformations and advanced type manipulations.

By using template literal types, developers can enforce stricter typing for string-based APIs. This leads to improved type safety and better code quality in TypeScript projects.

41. Working with JSON in TypeScript

TypeScript offers robust support for handling JSON data. JSON, or JavaScript Object Notation, is a popular data format used for storing and exchanging information.

TypeScript provides built-in methods for parsing JSON strings into JavaScript objects and converting objects back to JSON strings. The JSON.parse() method turns a JSON string into an object, while JSON.stringify() converts an object to a JSON string.

When working with JSON in TypeScript, developers can use interfaces to define the structure of JSON data. This helps ensure type safety and provides better autocomplete suggestions in code editors.

TypeScript also allows for type assertions when parsing JSON. This feature lets developers specify the expected type of parsed JSON data, improving code reliability and reducing errors.

For more complex JSON structures, TypeScript’s mapped types and conditional types can be useful. These advanced features enable developers to create flexible, reusable type definitions for JSON data.

Error handling is important when working with JSON in TypeScript. Try-catch blocks can be used to catch and handle parsing errors that may occur when dealing with invalid JSON data.

TypeScript’s strict null checks can help prevent common mistakes when working with potentially undefined or null values in JSON data. This feature prompts developers to handle these cases explicitly.

42. Symbols and Iterators

Symbols are a unique primitive data type in TypeScript. They create identifiers that are guaranteed to be unique. This makes them useful for object properties that should not conflict with other properties.

Iterators allow objects to define their iteration behavior. They work with for…of loops and other parts of JavaScript that expect iterable objects. An object is iterable if it has a Symbol.iterator property.

TypeScript supports both symbols and iterators. Developers can create custom iterators for their objects. This lets them control how their objects are looped over in for…of statements.

Symbols can be used as property keys in objects. This is helpful when you want to add properties to objects without worrying about name clashes. It’s especially useful in libraries or when working with objects you don’t fully control.

Iterators are closely linked with generators in TypeScript. Generators are functions that can pause and resume their execution. They automatically create iterators when called.

Understanding symbols and iterators is important for TypeScript developers. These features help create more flexible and powerful code. They’re often used in advanced programming patterns and libraries.

43. Understanding tsconfig.json

The tsconfig.json file is a key part of TypeScript projects. It sets up the compiler options and specifies which files to include or exclude.

This file sits in the root folder of a TypeScript project. It tells the TypeScript compiler how to turn TypeScript code into JavaScript.

Developers can set many options in tsconfig.json. These include the target JavaScript version, module system, and strict type checking rules.

The “compilerOptions” section is where most settings go. Here, you can turn on features like “strict” mode or set the output directory for compiled files.

The “include” and “exclude” arrays let you pick which files to compile. You can use patterns to match multiple files at once.

Some common options are “outDir” for setting the output folder, “rootDir” for the source folder, and “module” for choosing the module system.

The “target” option lets you pick which JavaScript version to compile to. This helps ensure your code works in older browsers if needed.

Using tsconfig.json makes it easy to share compiler settings across a team. It also helps IDEs understand your project structure.

44. Decorators with parameters

Decorators with parameters in TypeScript are a powerful way to customize how a decorator behaves. They allow developers to pass arguments to decorators, making them more flexible and reusable.

To create a decorator with parameters, you need to write a function that returns the decorator function. This outer function takes the parameters, while the inner function is the actual decorator.

Here’s a simple example of a decorator with parameters:

function log(prefix: string) {
  return function(target: any, key: string, descriptor: PropertyDescriptor) {
    const originalMethod = descriptor.value;
    descriptor.value = function(...args: any[]) {
      console.log(`${prefix}: ${key} called with`, args);
      return originalMethod.apply(this, args);
    };
    return descriptor;
  };
}

You can use this decorator like this:

class Calculator {
  @log('RESULT')
  add(a: number, b: number) {
    return a + b;
  }
}

When the add method is called, it will log the prefix, method name, and arguments before executing the original method.

Decorators with parameters are useful for adding configurable behavior to classes, methods, or properties. They can be used for logging, validation, caching, or any other cross-cutting concerns in your application.

Remember that decorator factories (functions that return decorators) are called when the code is defined, not when it’s run. This allows you to set up the decorator with the given parameters before it’s applied to the target.

45. Defining methods in Interfaces

Interfaces in TypeScript can include method definitions. These methods are like function signatures without the actual implementation.

To define a method in an interface, developers use a name followed by parentheses and a return type. The method body is not included, only the structure.

Here’s a simple example:

interface Greeter {
  greet(): void;
}

In this case, the interface defines a method called “greet” that doesn’t return anything.

Methods can also have parameters:

interface Calculator {
  add(a: number, b: number): number;
}

This interface defines an “add” method that takes two numbers and returns a number.

Classes that implement these interfaces must provide the actual method implementations. This ensures that objects of those classes have the expected behavior.

Interfaces can include optional methods too. These are marked with a question mark:

interface Logger {
  log(message: string): void;
  error?(message: string): void;
}

Here, the “error” method is optional. Classes implementing this interface must include the “log” method, but can choose to implement “error” or not.

Method definitions in interfaces help create consistent structures across different classes. They’re useful for defining contracts that classes must follow.

46. Overloads in TypeScript

Overloads in TypeScript let programmers define multiple function signatures for a single function. This feature helps create more flexible and type-safe code.

With function overloads, developers can specify different parameter types and return types for the same function name. This allows the function to handle various input types and produce appropriate outputs.

TypeScript checks the provided arguments against the overload signatures at compile-time. It selects the best-matching signature based on the input types.

To use overloads, developers declare multiple function signatures without a body. They then provide a single implementation that covers all the overloaded cases.

Overloads are useful when a function needs to accept different parameter combinations or return different types based on its inputs. They improve code readability and maintainability by clearly defining the expected inputs and outputs.

Function overloads help catch type-related errors early in the development process. They provide better IntelliSense support in IDEs, making it easier for developers to understand and use functions correctly.

When working with overloads, it’s important to list the most specific signatures first. This ensures TypeScript selects the correct overload when multiple signatures could match the provided arguments.

47. Dealing with DOM elements

TypeScript provides robust tools for working with DOM elements. It offers type definitions for HTML elements, making it easier to interact with the document structure.

Developers can use TypeScript to select elements, modify attributes, and handle events. The language’s static typing helps catch errors before runtime, improving code quality.

To select an element, TypeScript uses familiar methods like getElementById or querySelector. These methods return specific element types, allowing for better autocomplete and type checking.

Event handling in TypeScript is type-safe. Developers can specify the correct event type, ensuring proper event object properties are available.

TypeScript also supports creating and manipulating DOM elements programmatically. This includes adding, removing, or modifying elements and their attributes.

The language’s interface system allows developers to define custom element types. This is useful when working with web components or specialized DOM structures.

TypeScript’s strict null checks help prevent common DOM-related errors. It ensures that elements exist before attempting to access their properties or methods.

When dealing with forms, TypeScript provides type definitions for form elements and their values. This makes form handling and validation more reliable.

TypeScript’s generics can be used to create reusable DOM manipulation functions. These functions can work with different element types while maintaining type safety.

48. TypeScript Compiler API

The TypeScript Compiler API is a powerful tool for developers. It allows programmers to interact with the TypeScript compiler programmatically. This API provides access to the inner workings of the compiler.

Developers can use the Compiler API to analyze TypeScript code. It enables them to perform tasks like syntax parsing and type checking. This can be helpful for creating custom linting tools or code editors.

The API also allows for generating JavaScript from TypeScript code. This feature is useful for building transpilation tools or integrating TypeScript into existing build processes.

One key component of the Compiler API is the Abstract Syntax Tree (AST). The AST represents the structure of the code. Developers can traverse and manipulate this tree to perform advanced code analysis.

Another important aspect is the ability to create custom transformations. This allows developers to modify the code during the compilation process. It’s a useful feature for implementing code generation or automated refactoring tools.

The TypeScript Compiler API is commonly used in development tools and IDEs. It powers features like code completion, refactoring, and error checking. Many popular editors and build tools rely on this API for TypeScript support.

Learning to use the Compiler API requires a good understanding of TypeScript’s type system. It also demands familiarity with compiler design principles. Developers who master this API can create powerful tools to enhance TypeScript development workflows.

49. TypeScript performance optimization

TypeScript performance optimization focuses on improving code efficiency and execution speed. Developers can use several strategies to enhance TypeScript performance.

One key approach is to leverage TypeScript’s type system. By using precise types, developers can catch errors early and reduce runtime issues. This leads to more efficient code execution.

Another technique is to minimize the use of ‘any’ type. While convenient, ‘any’ bypasses TypeScript’s type checking, potentially leading to performance issues. Using specific types helps the compiler optimize code better.

Proper use of interfaces and type aliases can also boost performance. These features allow developers to define complex types without creating new objects, reducing memory usage.

Tree shaking is another useful optimization method. It removes unused code from the final bundle, resulting in smaller file sizes and faster load times.

Developers should also consider using const assertions for literal values. This allows TypeScript to infer more specific types, leading to better optimizations.

Lazy loading of modules can significantly improve initial load times. This technique loads modules only when needed, reducing the amount of code executed at startup.

Regular profiling and benchmarking help identify performance bottlenecks. Tools like Chrome DevTools can assist in pinpointing areas that need optimization.

Optimizing loops and reducing unnecessary function calls can also lead to performance gains. Using efficient data structures and algorithms is crucial for handling large datasets.

These optimization techniques can help create faster, more efficient TypeScript applications. Developers should apply them based on their specific project needs and performance goals.

50. TypeScript with Angular

Angular is a popular web application framework that uses TypeScript as its primary language. TypeScript brings strong typing and object-oriented features to Angular development.

When building Angular applications, TypeScript helps catch errors early in the development process. It provides better code completion and refactoring tools in IDEs.

Angular leverages TypeScript’s decorators to define components, services, and modules. These decorators add metadata to classes, making them more expressive and easier to understand.

TypeScript’s interfaces are useful for defining contracts between different parts of an Angular application. They help ensure consistency in data structures and API responses.

Angular’s dependency injection system benefits from TypeScript’s type annotations. This makes it easier to manage and inject services throughout the application.

TypeScript’s generics allow for creating reusable components and services in Angular. They provide type safety when working with collections and asynchronous operations.

The combination of TypeScript and Angular enables developers to build large-scale, maintainable applications. TypeScript’s static typing helps prevent common runtime errors and improves code quality.

Angular’s CLI (Command Line Interface) is built with TypeScript support in mind. It generates typed components, services, and other artifacts, streamlining the development process.

TypeScript’s module system integrates well with Angular’s architecture. It helps organize code into manageable, reusable pieces and supports lazy loading for better performance.

51. Testing TypeScript code

Testing TypeScript code is crucial for ensuring code quality and reliability. Developers can use various testing frameworks and tools designed specifically for TypeScript projects.

Jest is a popular choice for testing TypeScript applications. It offers built-in support for TypeScript and provides a simple, powerful testing environment.

Mocha is another widely used testing framework that works well with TypeScript. When combined with assertion libraries like Chai, it offers a flexible testing setup.

Unit tests are essential for verifying individual components or functions. These tests focus on small, isolated pieces of code to ensure they work as expected.

Integration tests check how different parts of the application work together. They help identify issues that may arise when combining multiple components.

End-to-end tests simulate real user scenarios, testing the entire application flow. Tools like Cypress or Selenium can be used for this type of testing.

TypeScript’s static typing helps catch errors early in the development process. This can reduce the number of runtime errors and make testing more efficient.

Test coverage tools like Istanbul can help developers identify areas of code that lack proper test coverage. This information guides the creation of additional tests.

Mocking and stubbing are important techniques in TypeScript testing. They allow developers to isolate the code being tested by replacing dependencies with controlled objects.

Continuous integration systems can automatically run tests whenever code changes are pushed. This practice helps catch issues early and maintains code quality throughout development.

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52. Explain generics in TypeScript with examples

Generics in TypeScript allow developers to create reusable code that works with different data types. They provide flexibility and type safety when working with various data structures.

Generics use angle brackets <> to define a type parameter. This parameter acts as a placeholder for the actual type that will be used when the code is executed.

A simple example of a generic function is an identity function:

function identity<T>(arg: T): T {
    return arg;
}

This function can work with any type. When called, TypeScript infers the type based on the argument passed:

let result = identity("Hello");  // Type is string
let num = identity(42);  // Type is number

Generics are also useful for creating flexible interfaces and classes:

interface Box<T> {
    value: T;
}

let stringBox: Box<string> = { value: "TypeScript" };
let numberBox: Box<number> = { value: 123 };

In this example, the Box interface can hold any type of value, providing type safety and flexibility.

53. How do you implement type guarding?

Type guarding in TypeScript helps check and narrow down types during runtime. It ensures that variables or objects are of a specific type before performing operations on them.

One common method is using the typeof operator. It checks primitive types like string, number, or boolean.

The instanceof operator is useful for checking if an object is an instance of a particular class or constructor function.

Custom type guards can be created with user-defined functions. These functions return a boolean and use the “is” keyword to tell TypeScript about the type.

The “in” operator checks if a property exists in an object. This can be helpful when working with union types or interfaces.

TypeScript also provides the “as” keyword for type assertions. While not a true type guard, it tells the compiler to treat a value as a specific type.

Pattern matching with switch statements can act as type guards in some cases. This is especially useful when working with discriminated unions.

Using these techniques, developers can write safer code that catches type-related errors early. Type guarding improves code reliability and makes debugging easier.

54. What is the purpose of union types?

Union types in TypeScript allow a variable to hold values of multiple types. They provide flexibility when working with data that could be of different types.

Union types are defined using the | symbol between the allowed types. For example, a variable could be either a string or a number:

let value: string | number;

This feature is useful when dealing with functions that can accept or return different types of data. It helps prevent errors by ensuring type safety while maintaining flexibility.

Union types also work well with conditional logic. Developers can check the type of a variable and handle each case accordingly:

if (typeof value === 'string') { console.log(value.toUpperCase()); } else { console.log(value.toFixed(2)); }

By using union types, TypeScript code becomes more robust and less prone to runtime errors. It allows for more precise type checking during development, catching potential issues early.

55. Differentiate between interface and type alias

Interfaces and type aliases are both ways to define custom types in TypeScript. They have some key differences that developers should know.

Interfaces are used to describe object shapes. They can be extended and merged with other interfaces. This makes them useful for creating reusable and expandable structures.

Type aliases can define any kind of type, not just objects. They can represent unions, intersections, and more complex types. This flexibility is helpful for creating aliases for existing types or combining multiple types.

Interfaces support declaration merging. This means you can add new properties to an existing interface. Type aliases don’t allow this kind of modification after they’re defined.

When it comes to extending, interfaces use the “extends” keyword. Type aliases use intersection types with “&” to achieve similar results. This difference affects how you combine and build upon existing types.

For object types, interfaces are often preferred. They’re easier to read and work well with tools that generate documentation. Type aliases are better for more complex type manipulations or when you need to create a new name for an existing type.

In practice, both interfaces and type aliases can often be used interchangeably for simple object types. The choice often comes down to personal or team preference.

56. How does TypeScript handle null and undefined?

TypeScript treats null and undefined as separate types. These types represent the absence of a value in different ways.

Undefined typically means a variable has been declared but not assigned a value. Null often indicates an intentional lack of value.

TypeScript offers several methods to handle null and undefined. The strict null checks compiler option helps catch potential errors related to these types.

Union types allow developers to explicitly state that a variable can be null or undefined. This approach ensures proper handling of these cases.

The optional chaining operator (?.) provides a safe way to access nested properties that might be null or undefined. It stops evaluation if it encounters a null or undefined value.

TypeScript’s non-null assertion operator (!) tells the compiler that a value is definitely not null or undefined. This should be used carefully to avoid runtime errors.

Developers can use type guards to narrow down types in conditional blocks. This helps TypeScript understand when a value is not null or undefined.

Proper handling of null and undefined in TypeScript leads to more robust code. It reduces the risk of runtime errors and improves overall type safety.

57. Illustrate the concept of decorators

Decorators in TypeScript are special functions that modify classes, methods, properties, or parameters. They use the @symbol syntax and run when the code is compiled.

A simple decorator might look like this:

function log(target: any, key: string) {
  console.log(`${key} was called`);
}

class Example {
  @log
  sayHello() {
    console.log("Hello!");
  }
}

In this example, the @log decorator logs when the sayHello method is called.

Decorators can also take arguments. Here’s an example of a decorator that sets a property:

function setProperty(value: string) {
  return function (target: any, key: string) {
    target[key] = value;
  };
}

class User {
  @setProperty("John")
  name: string;
}

This decorator sets the name property to “John” when the User class is created.

Decorators are useful for adding metadata, logging, or modifying behavior without changing the original code. They’re often used in frameworks and libraries to streamline common tasks.

58. Explain intersection types with examples

Intersection types in TypeScript let you combine multiple types into one. They are useful for creating a new type that has all the features of existing types.

To make an intersection type, use the ‘&’ symbol between the types you want to combine. This creates a new type with all the properties and methods of the combined types.

Here’s a simple example:

type Employee = {
  name: string;
  id: number;
};

type Manager = {
  department: string;
};

type ManagerEmployee = Employee & Manager;

In this case, ManagerEmployee has all the properties of both Employee and Manager. You can use it like this:

let boss: ManagerEmployee = {
  name: "Jane",
  id: 123,
  department: "Sales"
};

Intersection types are also handy with interfaces. You can combine interfaces to create more complex types:

interface Printable {
  print(): void;
}

interface Loggable {
  log(): void;
}

type PrintLogger = Printable & Loggable;

let myObject: PrintLogger = {
  print() { console.log("Printing..."); },
  log() { console.log("Logging..."); }
};

This PrintLogger type now has both print and log methods. Intersection types help create flexible and reusable code in TypeScript.

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59. What are mapped types used for?

Mapped types in TypeScript are used to create new types based on existing ones. They allow developers to transform the properties of an existing type in a structured way.

Mapped types help reduce code duplication by automating the creation of related types. They are especially useful when working with large or complex object types.

One common use of mapped types is to make all properties of an object optional. This can be helpful when creating partial versions of existing types.

Another application is to change the type of each property in an object. For example, developers can create a new type where all properties are readonly or converted to a different data type.

Mapped types also enable the creation of new types by picking or omitting specific properties from an existing type. This flexibility allows for precise control over type definitions.

In practical scenarios, mapped types are often used to generate utility types. These utility types can handle common transformations like making properties nullable or creating union types.

By using mapped types, developers can write more maintainable and type-safe code. They provide a powerful tool for manipulating and extending existing types in TypeScript projects.

60. How can you use conditional types effectively?

Conditional types in TypeScript allow developers to create flexible and reusable type definitions. They work by checking a condition and selecting a type based on the result.

One effective use of conditional types is to create more precise function overloads. This can help improve type inference and catch errors early in the development process.

Another way to use conditional types is for type narrowing. This technique allows for more specific type checking based on certain conditions, leading to safer and more predictable code.

Developers can also use conditional types to create mapped types. This enables the transformation of existing types into new ones based on specific criteria.

When working with generics, conditional types can be particularly powerful. They allow for the creation of generic types that adapt based on the input types provided.

It’s important to keep conditional types readable and maintainable. Breaking complex conditions into smaller, named types can help improve code clarity and reduce complexity.

Testing conditional types thoroughly is crucial. This ensures they behave as expected across different scenarios and edge cases.

By mastering conditional types, TypeScript developers can create more expressive and type-safe code, leading to fewer runtime errors and improved overall code quality.

61. Describe the keyof operator with examples

The keyof operator in TypeScript gets the keys of an object type as a union of string or numeric literal types. It’s useful for creating more flexible and type-safe code.

Here’s a simple example:

type User = {
  id: number;
  name: string;
  email: string;
};

type UserKeys = keyof User; // 'id' | 'name' | 'email'

In this case, UserKeys is a union type of ‘id’, ‘name’, and ’email’.

The keyof operator works with both interface and type alias definitions. It can be used with classes too:

class Person {
  name: string;
  age: number;
}

type PersonProps = keyof Person; // 'name' | 'age'

Keyof is often used with generics to create reusable utility types:

function getProperty<T, K extends keyof T>(obj: T, key: K): T[K] {
  return obj[key];
}

const user = { id: 1, name: 'Alice' };
const userName = getProperty(user, 'name'); // Type-safe access

This function ensures that the key passed is actually a key of the object, providing type safety.

62. What is the difference between readonly and const?

Readonly and const are both used in TypeScript to create immutable values, but they have some key differences.

Const is used for variables that cannot be reassigned after initial declaration. It applies to the variable itself, not its contents.

Readonly is used for object properties. It prevents modification of the property after the object is created.

With const, you can’t reassign the variable, but you can change the properties of an object it holds. Readonly prevents changing the property entirely.

Const is resolved at compile time, while readonly is enforced at runtime. This means const is slightly more efficient.

Readonly can be used with class properties, while const cannot. Const is typically used for standalone variables or object literals.

In practice, const is often used for values that should never change, like configuration settings. Readonly is useful for object properties that should be set once and not modified later.

Both readonly and const help prevent accidental changes to values, improving code safety and predictability. The choice between them depends on the specific use case and whether you’re working with variables or object properties.

63. How does TypeScript infer types during generic function creation?

TypeScript uses type inference to determine the types of generic functions automatically. This process relies on the context in which the function is used and the arguments passed to it.

When creating a generic function, TypeScript examines the input parameters and return values to deduce the appropriate types. It looks at how the function is called and what types of arguments are provided.

For example, if a generic function takes an array as input, TypeScript will infer the type of the array elements based on the actual array passed to the function. This saves developers from explicitly specifying types in many cases.

Type inference also works with more complex scenarios, such as nested generic types or functions that return other functions. TypeScript analyzes the entire function structure to make accurate type deductions.

In cases where TypeScript cannot infer types precisely, it may use broader types or require explicit type annotations. Developers can always provide type hints to guide the inference process if needed.

The infer keyword plays a role in advanced type inference scenarios, allowing the extraction of types from conditional statements within generic types. This feature enhances TypeScript’s ability to handle complex type relationships.

64. Explain how to set up a TypeScript project

Setting up a TypeScript project involves a few key steps. First, developers need to install Node.js and npm (Node Package Manager) on their system.

Next, they create a new project folder and navigate to it in the terminal. They can then initialize a new npm project by running “npm init -y”.

Installing TypeScript is the next step. This is done by running “npm install typescript –save-dev” in the terminal.

After installation, developers create a TypeScript configuration file named “tsconfig.json”. This file specifies compiler options and project settings.

The “tsconfig.json” file can be generated automatically by running “npx tsc –init” in the terminal.

Developers then set up their project structure by creating folders for source files and compiled output. Common folders include “src” for TypeScript files and “dist” for compiled JavaScript.

To compile TypeScript files, the command “npx tsc” is used. This converts TypeScript code into JavaScript based on the settings in “tsconfig.json”.

For easier development, many developers add scripts to their “package.json” file. These scripts can automate tasks like compilation and running the project.

Lastly, developers can set up their preferred code editor or IDE with TypeScript support for features like syntax highlighting and auto-completion.

65. What is the utility of the ‘as’ keyword?

The ‘as’ keyword in TypeScript is used for type assertions. It tells the compiler to treat a value as a specific type, even if TypeScript can’t verify it at compile time.

Type assertions are useful when developers have more information about a value’s type than TypeScript does. This often happens when working with DOM elements or third-party libraries.

The ‘as’ keyword helps avoid type errors in situations where TypeScript’s type inference is not sufficient. It allows programmers to override the compiler’s type checking.

Here’s an example:

let someValue: any = "hello";
let strLength: number = (someValue as string).length;

In this case, ‘as string’ tells TypeScript to treat ‘someValue’ as a string, allowing access to the ‘length’ property.

It’s important to note that type assertions don’t change the runtime behavior of code. They only affect how TypeScript analyzes the code during compilation.

While ‘as’ is helpful, it should be used carefully. Overusing it can lead to runtime errors if the assertions are incorrect.

66. How do you use namespaces in TypeScript?

Namespaces in TypeScript group related code under a single name. They help organize and prevent naming conflicts in large projects.

To create a namespace, use the “namespace” keyword followed by a name. Inside the namespace, define functions, classes, or interfaces.

Here’s a simple example:

namespace MyNamespace {
  export function sayHello(name: string) {
    console.log(`Hello, ${name}!`);
  }
}

The “export” keyword makes the function accessible outside the namespace.

To use the function from another file, you need to reference the namespace:

/// <reference path="myNamespace.ts" />
MyNamespace.sayHello("Alice");

Alternatively, use the import statement:

import { MyNamespace } from "./myNamespace";
MyNamespace.sayHello("Bob");

Namespaces can be nested for more complex organization. They’re useful for creating modular code and avoiding global scope pollution.

While namespaces are still supported, many developers now prefer using ES6 modules for better compatibility with modern JavaScript practices.

67. Discuss the utility types Partial and Required

TypeScript offers utility types that help modify existing types. Two important ones are Partial and Required.

Partial makes all properties of a type optional. It’s useful when you want to create an object with only some properties of a type.

For example, if you have a User type with name and age, Partial lets you make an object with just the name or just the age.

Required does the opposite. It makes all properties of a type mandatory. This is helpful when you want to ensure all fields are filled.

If you have a type with optional fields, using Required forces all those fields to be present.

These utility types save time and reduce errors. They let you work with flexible object structures without writing new types from scratch.

Partial is great for update functions where you might only change some fields. Required works well for validation to make sure all data is present.

Using these types makes your code more robust and easier to maintain. They help catch errors early and make your intentions clear to other developers.

68. What are module augmentations?

Module augmentations in TypeScript allow developers to extend existing modules with new functionality. This feature is particularly useful when working with third-party libraries or legacy code.

With module augmentation, programmers can add new properties, methods, or types to existing modules without modifying the original source code. This approach keeps the codebase clean and organized.

To use module augmentation, developers declare a module with the same name as the one they want to extend. Inside this declaration, they can add new elements to the module.

This technique is especially helpful when developers need to add custom functionality to external libraries. It allows them to enhance these libraries while maintaining compatibility with future updates.

Module augmentation provides a way to split module definitions across multiple files. This can improve code organization and make large projects more manageable.

It’s important to note that module augmentation only works with modules that use the export keyword. It cannot be used to modify global scope or non-module scripts.

When using module augmentation, developers should be careful not to introduce conflicts with existing module members. It’s best to check the original module’s documentation to avoid any potential issues.

69. How do you create and emit custom events in TypeScript

Custom events in TypeScript allow components to communicate without tight coupling. To create a custom event, developers use the CustomEvent constructor from the DOM API.

Here’s a basic example of creating and emitting a custom event:

const myEvent = new CustomEvent('myCustomEvent', { detail: { message: 'Hello!' } });
document.dispatchEvent(myEvent);

To listen for this event, you can use an event listener:

document.addEventListener('myCustomEvent', (event: CustomEvent) => {
  console.log(event.detail.message);
});

For type safety, it’s useful to declare custom event types. This can be done by extending the existing EventMap interface:

declare global {
  interface EventMap {
    'myCustomEvent': CustomEvent<{ message: string }>;
  }
}

This ensures type checking when working with custom events. Emitting events can also be done using a custom EventEmitter class:

class MyEmitter {
  private listeners: { [key: string]: Function[] } = {};

  on(event: string, callback: Function) {
    if (!this.listeners[event]) {
      this.listeners[event] = [];
    }
    this.listeners[event].push(callback);
  }

  emit(event: string, data: any) {
    if (this.listeners[event]) {
      this.listeners[event].forEach(callback => callback(data));
    }
  }
}

This approach provides more control over event handling and is useful for non-DOM contexts.

70. Explain the structural type system in TypeScript

TypeScript uses a structural type system. This means it focuses on the shape of an object rather than its declared type.

In a structural type system, two types are considered the same if they have the same structure. This applies even if they have different names.

For example, if two objects have the same properties and methods, TypeScript treats them as compatible. This is true even if they were created from different classes or interfaces.

This system allows for more flexible code. It lets developers create objects that fit a certain shape without explicitly implementing an interface.

The structural type system also enables duck typing. This concept states that if an object looks and behaves like a duck, it can be treated as a duck.

This approach differs from nominal typing used in some other languages. In nominal typing, two types are only considered the same if they are explicitly declared as the same type.

TypeScript’s structural typing makes the language more JavaScript-like. It allows for easier integration with existing JavaScript code and libraries.

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71. Why is TypeScript considered a superset of JavaScript?

TypeScript builds on JavaScript by adding new features and capabilities. It includes all of JavaScript’s syntax and functionality, making it a true superset.

Any valid JavaScript code can run as TypeScript without changes. This allows developers to gradually adopt TypeScript in existing projects.

TypeScript introduces optional static typing. This helps catch errors early and improves code quality. It also enables better tooling support and autocompletion in editors.

The language adds object-oriented programming features like classes and interfaces. These make it easier to build and maintain large-scale applications.

TypeScript also supports newer JavaScript features before they’re widely available in browsers. It compiles these features down to older versions of JavaScript for compatibility.

By extending JavaScript’s capabilities, TypeScript aims to make development more efficient and scalable. It provides a smoother path for building complex applications while maintaining JavaScript compatibility.

72. What are discriminated unions?

Discriminated unions are a powerful feature in TypeScript. They allow developers to create a type that can represent multiple different types of values.

Each value in a discriminated union has a unique property called a “discriminant.” This property helps identify which specific type the value belongs to.

Discriminated unions are also known as tagged unions or algebraic data types. They provide a way to handle different types under a common interface.

To create a discriminated union, TypeScript uses the ‘|’ operator to combine a set of types. Each type in the union typically has a shared property with a literal type.

This feature is useful for writing more robust and type-safe code. It helps prevent errors by ensuring that the correct properties are accessed for each type.

Discriminated unions make it easier to work with complex data structures. They allow TypeScript to infer types based on the discriminant property.

When used correctly, discriminated unions can lead to cleaner and more maintainable code. They provide a structured way to handle different cases in a type-safe manner.

73. How does the import type syntax work?

The import type statement is a feature in TypeScript that allows developers to import only type information from a module. This syntax was introduced in TypeScript 3.8.

When using import type, no JavaScript code is imported at runtime. It’s purely for type-checking purposes.

Here’s an example of how to use import type:

import type { Person } from './person';

In this case, only the Person type is imported, not any actual code from the person module.

This feature is useful when you want to use types from another module without importing any values or functions. It can help reduce bundle size in some cases.

The import type syntax can also be used with entire modules:

import type * as PersonModule from './person';

This imports all types from the PersonModule, but no runtime code.

It’s important to note that regular import statements can still import both types and values. The import type syntax is specifically for cases where you only need the type information.

74. How can you ensure type safety in asynchronous code?

TypeScript offers several ways to maintain type safety in asynchronous operations. One key approach is using Promises with proper type annotations. Developers can specify the expected return type of a Promise, allowing TypeScript to check for type consistency.

Another effective method is leveraging async/await syntax. This approach simplifies asynchronous code and makes it easier to handle errors while preserving type information. TypeScript can infer the types of awaited values, enhancing code reliability.

Generic types also play a crucial role in ensuring type safety for asynchronous functions. By using generics, developers can create flexible yet type-safe functions that work with various data types.

Type guards are useful for narrowing down types in asynchronous code. They help TypeScript understand the specific type of a value after certain checks, improving type inference and reducing errors.

The TypeScript compiler’s strict mode can catch potential type-related issues in asynchronous code. Enabling strict null checks and strict function types helps identify and prevent common pitfalls.

Using interfaces or type aliases to define the structure of asynchronous function parameters and return values enhances code clarity and type safety. This practice makes it easier to spot inconsistencies and maintain a robust type system throughout the codebase.

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75. What is the purpose of DefinitelyTyped?

DefinitelyTyped is a popular repository that houses declaration files for numerous JavaScript libraries. Its main purpose is to provide TypeScript type definitions for JavaScript code that lacks built-in type information.

Many JavaScript libraries are not written in TypeScript and don’t include their own type declarations. This can make it challenging to use these libraries in TypeScript projects.

DefinitelyTyped solves this problem by offering a centralized collection of community-contributed type definitions. These definitions allow TypeScript developers to use JavaScript libraries with full type checking and autocompletion.

The repository contains thousands of type declaration files for various JavaScript libraries and frameworks. This extensive collection helps developers integrate external JavaScript code into their TypeScript projects seamlessly.

By using DefinitelyTyped, developers can enjoy the benefits of static typing even when working with libraries that don’t natively support TypeScript. This improves code quality, reduces errors, and enhances the development experience.

76. Illustrate enums with examples

Enums in TypeScript allow developers to define a set of named constants. They make code more readable and help catch errors at compile-time.

Here’s a simple enum example:

enum Color {
  Red,
  Green,
  Blue
}

let myColor: Color = Color.Green;

In this case, Red is 0, Green is 1, and Blue is 2. Developers can also set custom values:

enum Direction {
  Up = "UP",
  Down = "DOWN",
  Left = "LEFT",
  Right = "RIGHT"
}

let playerDirection: Direction = Direction.Left;

Enums can be useful for representing a fixed set of options:

enum DaysOfWeek {
  Monday,
  Tuesday,
  Wednesday,
  Thursday,
  Friday,
  Saturday,
  Sunday
}

function isWeekend(day: DaysOfWeek): boolean {
  return day === DaysOfWeek.Saturday || day === DaysOfWeek.Sunday;
}

This example shows how enums can make code more expressive and easier to understand. They provide a way to work with a predefined set of values, improving code clarity and reducing errors.

77. How do you handle versioning in TypeScript projects?

TypeScript projects use semantic versioning to manage different releases. This system uses three numbers: major, minor, and patch versions.

The major version changes when there are big updates that may break existing code. Minor versions add new features without breaking older ones. Patch versions fix bugs or make small improvements.

Developers often use package managers like npm or Yarn to handle versioning. These tools allow easy updating of project dependencies to specific versions.

The package.json file in a TypeScript project lists all dependencies and their version numbers. This file helps keep track of which versions are being used.

Version control systems like Git also play a role in managing TypeScript project versions. Developers can create tags or branches for different releases.

For larger projects, teams might use a monorepo setup. This approach helps manage multiple related packages and their versions together.

Testing is crucial when updating versions. Automated tests can catch issues that might arise from version changes.

TypeScript itself has its own version number. Projects should specify which TypeScript version they use to avoid compatibility problems.

78. Explain ‘this’ typing in TypeScript

TypeScript allows developers to specify the type of ‘this’ in functions and methods. This feature helps catch errors related to incorrect usage of ‘this’ and provides better intellisense.

In TypeScript, you can declare the type of ‘this’ as the first parameter of a function. This parameter is special and doesn’t affect the function’s call signature.

Here’s an example:

function greet(this: { name: string }) {
  console.log(`Hello, ${this.name}!`);
}

This typing ensures that ‘this’ inside the function has a ‘name’ property of type string.

When using ‘this’ typing with classes, TypeScript automatically infers the correct type. For methods, ‘this’ refers to the instance of the class.

In arrow functions, ‘this’ is captured from the surrounding context. TypeScript preserves this behavior and doesn’t allow explicit ‘this’ parameter declarations for arrow functions.

‘This’ typing is particularly useful when working with callbacks or functions that will be used as methods on objects with specific shapes.

By leveraging ‘this’ typing, developers can write more robust code and catch potential runtime errors during compilation.

79. What are specialized signatures?

Specialized signatures in TypeScript are function overloads that provide more specific type information for different parameter combinations. They allow developers to define multiple function signatures for a single function implementation.

These signatures help TypeScript understand how a function behaves with different input types. They improve type checking and provide better intellisense in code editors.

To use specialized signatures, developers list multiple function declarations without bodies. These are followed by a single implementation that covers all cases.

This feature is especially useful when a function can accept different parameter types or return different types based on its inputs. It allows for more precise type definitions and safer code.

Specialized signatures make code more readable and self-documenting. They clearly show the expected inputs and outputs for each function variation.

By using this technique, developers can create more flexible and type-safe functions. This leads to fewer runtime errors and improved code quality overall.

80. Discuss the utility of template literal types

Template literal types in TypeScript let developers create flexible and precise string-based types. They work like template literals in JavaScript, but for types instead of values.

These types are useful for making string manipulation safer and more powerful. They can combine existing string types to form new ones, helping catch errors early in development.

One key benefit is the ability to create unions of string types automatically. This saves time and reduces the chances of typos or mistakes when defining allowed string values.

Template literal types also enable pattern matching on strings. This feature is handy for tasks like parsing URLs or validating input formats without using regular expressions.

They can enforce naming conventions or structure in code. For example, they can ensure that certain strings always start or end with specific patterns.

When used with mapped types, template literal types allow for dynamic creation of property names. This capability is especially useful when working with APIs or complex data structures.

These types integrate well with TypeScript’s type inference system. They help provide better autocomplete suggestions and more accurate type checking throughout a codebase.

81. How can you implement dependency injection in TypeScript?

Dependency injection in TypeScript helps create more flexible and testable code. It involves providing a class with its dependencies from external sources rather than creating them internally.

To implement dependency injection, start by defining interfaces for your dependencies. This allows you to work with abstractions rather than concrete implementations.

Next, create classes that implement these interfaces. These classes will provide the actual functionality needed by other parts of your application.

In the class that needs the dependencies, declare them as parameters in the constructor. This makes the dependencies explicit and allows them to be easily injected.

Use a DI container or framework to manage the creation and injection of dependencies. Popular options for TypeScript include InversifyJS and tsyringe.

When using a DI container, register your dependencies with the container. This tells the container how to create instances of your classes and their dependencies.

To use the injected dependencies, simply access them through the class properties where they were assigned in the constructor.

By following these steps, you can create a more modular and maintainable codebase. Dependency injection makes it easier to swap out implementations and test individual components in isolation.

82. Describe advanced types and their uses

TypeScript offers several advanced types that give developers more control and flexibility. Union types allow variables to hold values of multiple types. For example, a variable could be either a string or a number.

Intersection types combine multiple types into one. This is useful for creating complex objects that have properties from different interfaces.

Conditional types let developers create types that depend on other types. They use an if-then structure to determine the resulting type based on a condition.

Mapped types transform existing types into new ones. They can add, remove, or modify properties of an original type.

Index types enable the use of dynamic property names. This is helpful when working with objects that have unknown property names.

Literal types restrict variables to specific values. They can be strings, numbers, or boolean values.

These advanced types help catch errors early and make code more precise. They also improve code readability and make it easier to understand the expected data structures.

Developers use these types to create more robust and maintainable code. They are especially useful in large projects where type safety is crucial.

83. How do you use function overloads?

Function overloads in TypeScript let you define multiple ways to call a function. This is useful when a function can handle different types or numbers of arguments.

To use function overloads, you first declare the function signatures. These show the possible parameter types and return types the function can have.

Next, you write the implementation signature. This is the actual function body that handles all the overloaded cases.

Here’s a simple example:

function greet(name: string): string;
function greet(age: number): string;
function greet(value: string | number): string {
  if (typeof value === "string") {
    return `Hello, ${value}!`;
  } else {
    return `You are ${value} years old.`;
  }
}

In this example, greet can take either a string or a number. The function body checks the type of the input and responds accordingly.

When using the function, TypeScript will check that you’re calling it correctly based on the overload signatures. This helps catch errors and provides better autocomplete suggestions.

84. What is the concept of ambient namespaces?

Ambient namespaces in TypeScript allow developers to describe the shape of existing JavaScript code. They provide a way to declare the types and structures of external libraries or modules that don’t have TypeScript definitions.

Developers use ambient namespaces when working with global objects or libraries that are not modular. These namespaces help TypeScript understand the structure of external code without actually implementing it.

To create an ambient namespace, developers use the ‘declare’ keyword followed by ‘namespace’. Inside the namespace, they can define interfaces, classes, and other TypeScript constructs that represent the external code.

Ambient namespaces are useful for adding type information to legacy JavaScript libraries. They enable better type checking and autocompletion in TypeScript projects that use these libraries.

One key benefit of ambient namespaces is that they don’t generate any JavaScript output. They exist solely for TypeScript’s type system and are removed during compilation.

Developers often use ambient namespaces in declaration files (.d.ts) to provide type information for libraries that lack TypeScript support. This helps maintain type safety when using external JavaScript code in TypeScript projects.

85. Discuss any potential pitfalls with TypeScript decorators

TypeScript decorators can be tricky to use correctly. One common issue is that they can impact performance if overused. Adding too many decorators to a class or method can slow down the program.

Another pitfall is that decorators are not yet standardized in JavaScript. This means their behavior might change in future TypeScript versions. Developers should be cautious when relying heavily on decorators in production code.

Decorators can also make code harder to understand. They add an extra layer of complexity that might confuse other developers working on the project. This can lead to maintenance issues down the line.

Testing code with decorators can be challenging. The decorated functions might behave differently than expected, making it difficult to write accurate unit tests.

Some developers might be tempted to use decorators for tasks better suited to other programming patterns. This can result in overly complex or inefficient code.

Debugging decorated code can be more difficult. The stack trace might not clearly show where an error is occurring, especially with multiple decorators applied to a single element.

Lastly, decorators can sometimes interfere with TypeScript’s type inference. This may require developers to add explicit type annotations, reducing some of TypeScript’s benefits.

86. How can you utilize Awaited utility type?

The Awaited utility type in TypeScript helps developers work with asynchronous code more effectively. It unwraps the type inside a Promise, making it easier to handle the resolved value.

Developers can use Awaited to define the expected type of an awaited Promise. This is particularly useful when working with functions that return Promises or when dealing with nested Promises.

For example, if a function returns Promise, using Awaited<Promise> will result in the type string. This saves developers from manually unwrapping Promise types.

The Awaited type is also helpful when working with APIs or libraries that return complex Promise structures. It simplifies type annotations and improves code readability.

When used with generic types, Awaited can provide more precise type information for async functions. This leads to better type inference and fewer explicit type annotations.

Awaited can be combined with other utility types to create more advanced type transformations. This flexibility allows developers to handle various asynchronous scenarios in their TypeScript code.

By using Awaited, developers can catch potential type errors related to Promise handling at compile-time. This results in more robust and type-safe code when working with asynchronous operations.

87. What are assertion functions and their uses?

Assertion functions in TypeScript help check if certain conditions are true at runtime. They’re useful for verifying assumptions about code and catching errors early.

These functions tell TypeScript that a specific condition is always true after they’re called. If the condition isn’t met, they throw an error.

Developers use assertion functions to narrow down types. This makes the code safer and easier to understand. They’re especially handy when TypeScript can’t figure out types on its own.

One common use is to check if a value isn’t null or undefined before using it. This prevents errors that might happen if the value is missing.

Assertion functions also help with testing. They make sure important conditions are met during program execution. This catches bugs that might otherwise slip through.

In libraries, assertion functions can validate input. They ensure that users provide the right kind of data, making the library more reliable.

By using assertion functions, developers can write clearer, more robust code. They provide an extra layer of safety and help catch mistakes early in the development process.

88. Outline ways to improve compilation performance

TypeScript compilation can be sped up through several methods. One approach is to use the –incremental flag, which saves information from the last compilation to speed up future builds.

Another technique is to employ project references. This allows splitting a large project into smaller parts that can be built separately, reducing overall compilation time.

Upgrading to the latest TypeScript version often brings performance improvements. Each new release typically includes optimizations that can speed up the compilation process.

Limiting the use of const enums can also help. While useful, they require the compiler to do more work, potentially slowing down compilation.

Avoiding excessive use of complex types and type inference can boost performance. Simpler type structures are generally faster for the compiler to process.

Using the –skipLibCheck flag can speed up compilation by skipping type-checking of declaration files. This is especially useful for large projects with many dependencies.

Optimizing tsconfig.json settings can make a difference. Carefully choosing which files to include and exclude can prevent unnecessary compilation of irrelevant code.

Leveraging TypeScript’s watch mode during development can provide faster feedback. This mode only recompiles changed files, leading to quicker build times.

89. How does TypeScript support JSX?

TypeScript has built-in support for JSX, a syntax extension used in React. It allows developers to write HTML-like code within JavaScript files.

To use JSX in TypeScript, files must have a .tsx extension. This tells the compiler to expect JSX syntax.

TypeScript can check JSX for errors and provide autocompletion. It understands the structure of JSX elements and their props.

Developers can define custom JSX elements using interfaces. This enables TypeScript to validate prop types for these elements.

The TypeScript compiler can transform JSX into regular JavaScript. It offers different output modes, like ‘react’ or ‘preserve’.

TypeScript also supports type checking for React components. It can infer prop types and state types automatically in many cases.

For stricter type checking, developers can use generic types with React components. This ensures better type safety for props and state.

TypeScript’s JSX support integrates well with popular React libraries and tools. It works seamlessly with code editors and build systems.

90. How can currying be used in TypeScript?

Currying is a technique in TypeScript that breaks down a function with multiple parameters into a series of nested functions. Each nested function takes a single argument.

This approach allows for creating more specialized versions of functions. It’s useful when you want to reuse a function with some pre-set parameters.

In TypeScript, currying can be implemented using arrow functions and closures. Here’s a basic example:

const add = (a: number) => (b: number) => a + b; const addFive = add(5); console.log(addFive(3)); // Output: 8

In this case, ‘add’ is a curried function. It takes one argument and returns another function that takes the second argument.

Currying can enhance code reusability and flexibility. It allows partial application of a function’s arguments, which can be helpful in scenarios where not all parameters are available at once.

TypeScript’s type system works well with curried functions. It can infer and check types at each step, ensuring type safety throughout the currying process.

Developers can use currying to create more modular and composable code. It’s particularly useful in functional programming patterns and can lead to cleaner, more maintainable codebases.

91. Explain module resolution strategies

Module resolution strategies are methods TypeScript uses to find and load modules. These strategies determine how the compiler locates imported modules in a project.

TypeScript has two main module resolution strategies: Classic and Node. The Classic strategy is simple but less flexible. It looks for files with .ts, .tsx, or .d.ts extensions relative to the importing file.

The Node strategy is more advanced and follows Node.js module resolution rules. It searches for modules in node_modules folders and supports package.json files. This strategy is the default in TypeScript projects.

TypeScript also allows customizing module resolution through tsconfig.json settings. The “moduleResolution” option lets developers choose between different strategies.

Another important aspect is path mapping. This feature allows defining custom module paths, making imports more flexible and easier to manage in large projects.

Understanding module resolution is crucial for TypeScript developers. It helps in organizing code, managing dependencies, and troubleshooting import issues effectively.

92. What is backward compatibility in TypeScript and why does it matter?

Backward compatibility in TypeScript means newer versions of the language can work with code written in older versions. This feature allows developers to update TypeScript without breaking existing projects.

TypeScript uses semantic versioning to manage changes. Major versions may introduce breaking changes, while minor and patch versions maintain compatibility.

Backward compatibility matters because it helps developers adopt new TypeScript features gradually. They can update their tools without rewriting large portions of code.

It also ensures that libraries and frameworks built with older TypeScript versions continue to function in newer environments. This stability is crucial for maintaining large codebases and supporting long-term projects.

TypeScript’s approach to backward compatibility includes careful consideration of syntax changes. The language designers aim to minimize disruptions to existing code when introducing new features.

Developers can use configuration options to control compatibility levels. This flexibility allows teams to balance using new features with maintaining existing code.

While TypeScript strives for backward compatibility, some changes may be necessary for language improvements. In these cases, the TypeScript team provides migration guides and tools to help developers update their code smoothly.

93. How do you handle type casting issues?

Type casting in TypeScript helps developers change a variable from one type to another. This can be useful when working with different data types or when integrating with JavaScript code.

One common method is using the “as” keyword. This tells TypeScript to treat a variable as a specific type. For example, you can cast a variable to a string like this: (someVariable as string).

Another approach is using angle brackets. You can write someVariable to cast a variable to a string type. This method is less common but still valid in many situations.

Sometimes, type assertions may not be enough. In these cases, you might need to use type guards. Type guards are functions that check the type of a variable at runtime.

It’s important to be careful when casting types. Incorrect casting can lead to runtime errors. Always make sure the variable can actually be of the type you’re casting it to.

When dealing with unknown types, it’s often safer to use the “unknown” type first. Then, you can narrow it down using type guards or assertions as needed.

Remember, TypeScript’s static type checking helps catch many issues at compile time. But type casting can bypass these checks, so use it judiciously.

94. Illustrate how the module system works in TypeScript

TypeScript’s module system allows developers to organize code into reusable units. It helps manage dependencies and scope in large applications.

Modules in TypeScript are similar to JavaScript modules. They can export functions, classes, or variables that other modules can import and use.

To create a module, developers use the ‘export’ keyword. This makes the code available for use in other files. Here’s an example:

// math.ts
export function add(a: number, b: number): number {
  return a + b;
}

To use this module in another file, the ‘import’ keyword is used:

// app.ts
import { add } from './math';

console.log(add(5, 3)); // Output: 8

TypeScript supports different module formats like CommonJS, AMD, and ES6. The compiler can output code in various module systems based on the target environment.

Modules can also have a default export. This is useful when a module primarily exports one thing:

// greeter.ts
export default function greet(name: string): string {
  return `Hello, ${name}!`;
}

Importing a default export looks like this:

// app.ts
import greet from './greeter';

console.log(greet('Alice')); // Output: Hello, Alice!

TypeScript’s module system helps create cleaner, more maintainable code by promoting encapsulation and reducing global namespace pollution.

95. How can type compatibility benefit you in TypeScript?

Type compatibility in TypeScript allows for flexible coding practices. It lets developers use types that are similar but not exactly the same.

This feature helps when working with different libraries or codebases. It enables smoother integration of various components without strict type matching.

Type compatibility makes code reuse easier. Developers can create functions or classes that work with multiple similar types. This reduces the need for duplicate code.

It also helps with gradual typing. When converting JavaScript to TypeScript, type compatibility allows for a smoother transition. Existing code can work alongside new TypeScript code more easily.

Type compatibility supports structural typing. This means TypeScript focuses on the shape of an object rather than its exact type name. It allows for more intuitive coding patterns.

In large projects, type compatibility can speed up development. It reduces the need for excessive type annotations and explicit type conversions. This leads to cleaner, more readable code.

Overall, type compatibility in TypeScript strikes a balance between type safety and coding flexibility. It helps developers write more efficient and adaptable code.

96. What are the best practices for structuring TypeScript applications?

TypeScript applications benefit from a well-organized structure. One common approach is to group files by feature or module rather than by type.

This method keeps related code together, making it easier to navigate and maintain. It’s also helpful to use clear, descriptive names for files and folders.

Separating concerns is another key practice. Keep business logic, UI components, and data access in distinct layers. This separation improves code reusability and testing.

Using interfaces to define shapes of objects and function signatures enhances type safety. It’s wise to create these interfaces in separate files for better organization.

Consistent naming conventions for files, classes, and functions improve readability. Following a style guide, like the official TypeScript Style Guide, ensures uniformity across the project.

Leveraging TypeScript’s module system helps manage dependencies. Importing only what’s needed reduces bundle size and improves performance.

Proper error handling is crucial. Implementing custom error types and using try-catch blocks where appropriate enhances application robustness.

Regular code reviews and automated testing are vital practices. They help catch issues early and maintain code quality as the application grows.

97. Discuss the concept of type evolution

Type evolution in TypeScript refers to how types can change over time as code is developed and modified. It’s an important concept for experienced developers to understand.

Types in TypeScript are not static. They can evolve as the codebase grows and requirements change. This flexibility allows developers to adapt their types to new needs.

One way types evolve is through refinement. As more information becomes available, developers can narrow down types to be more specific. This leads to better type safety and clearer code.

Another aspect of type evolution is expansion. Sometimes, types need to accommodate new properties or values. TypeScript allows developers to extend existing types to include new features.

Type evolution also involves refactoring. As code patterns emerge, developers might create new types or combine existing ones. This helps maintain a clean and organized type system.

Interfaces play a key role in type evolution. They can be extended and merged, allowing for gradual changes to types over time. This is especially useful in large codebases.

Understanding type evolution helps developers write more maintainable and scalable TypeScript code. It allows for adapting to changing requirements without sacrificing type safety.

98. What are the advantages and disadvantages of using TypeScript?

TypeScript offers several advantages over JavaScript. It adds static typing, which helps catch errors early in the development process. This can lead to more robust and maintainable code.

TypeScript provides better code organization through interfaces, enums, and modules. These features make it easier to structure large projects and work in teams.

The language also offers improved tooling support. IDEs can provide more accurate autocompletion and refactoring tools when working with TypeScript.

TypeScript is a superset of JavaScript, meaning any valid JavaScript code is also valid TypeScript. This allows for gradual adoption in existing projects.

On the flip side, TypeScript has some drawbacks. It requires compilation before running, which adds an extra step to the development process.

The learning curve can be steeper for developers new to static typing. It may take time to understand and effectively use TypeScript’s additional features.

TypeScript files tend to be larger than their JavaScript counterparts due to type annotations. This can slightly increase project size and load times.

Some third-party libraries may lack TypeScript definitions, requiring extra work to integrate them into a TypeScript project.

Despite these challenges, many developers find that TypeScript’s benefits outweigh its disadvantages, especially for larger projects or teams.

99. How can loop types be implemented in TypeScript?

TypeScript supports several loop types for iterating through data structures. The most common loop types are for, while, and do-while loops.

For loops are useful when you know how many times you want to repeat a block of code. They have three parts: initialization, condition, and increment/decrement.

While loops run as long as a specified condition is true. They check the condition before each iteration, making them suitable for situations where you don’t know how many times the loop will run.

Do-while loops are similar to while loops, but they always execute the code block at least once before checking the condition.

TypeScript also offers for…of loops for iterating over arrays and other iterable objects. These loops provide a simpler syntax for accessing array elements directly.

For…in loops are used to iterate over the properties of an object. They are helpful when you need to work with object keys.

TypeScript’s type system enhances these loops by providing better type checking and autocompletion. This helps catch errors early and improves code quality.

When working with arrays, TypeScript can infer the type of loop variables, making the code more concise and less prone to errors.

100. What is the importance of TypeScript’s type system in a large project?

TypeScript’s type system plays a crucial role in large projects. It helps catch errors early in the development process. This leads to fewer bugs making it to production.

The type system improves code quality and readability. It makes the code easier to understand and maintain. This is especially valuable when multiple developers work on the same project.

TypeScript’s type checking enables better autocompletion and IntelliSense in IDEs. This boosts developer productivity and reduces mistakes. It also makes refactoring safer and more efficient.

In large projects, the type system helps enforce consistent interfaces between different parts of the application. This is vital for creating scalable and modular codebases.

The type system also serves as a form of documentation. It clearly shows the expected input and output of functions and methods. This makes it easier for new team members to understand and work with existing code.

TypeScript’s structural typing allows for flexible yet type-safe code. This balance is particularly useful in large projects where adaptability is key. It enables teams to write robust code that can evolve with changing requirements.

101. How to manage TypeScript’s configuration?

TypeScript configuration is managed through a file called tsconfig.json. This file is placed in the root directory of a TypeScript project. It specifies compiler options and project settings.

To create a tsconfig.json file, developers can use the command “tsc –init” in the terminal. This generates a file with default settings.

The tsconfig.json file contains various options to control TypeScript compilation. Some key options include “target” for specifying the output JavaScript version, “module” for setting the module system, and “strict” for enabling strict type-checking.

Developers can customize these options based on project needs. For example, they might set “outDir” to specify where compiled files should be placed, or “rootDir” to define the source files location.

The “include” and “exclude” fields in tsconfig.json help manage which files are part of the compilation process. “Include” lists files to be compiled, while “exclude” specifies files to ignore.

For larger projects, developers can use “extends” in tsconfig.json to inherit configurations from a base file. This promotes consistency across multiple sub-projects.

IDEs and text editors often use tsconfig.json to provide better TypeScript support, including autocompletion and error checking.

102. What does structural typing mean in TypeScript?

Structural typing is a key feature in TypeScript’s type system. It focuses on the shape of objects rather than their explicit type declarations.

In structural typing, two types are considered compatible if they have the same structure. This means they share the same properties and methods, regardless of their names or how they were defined.

TypeScript compares the structures of objects to determine if they match. If an object has all the required properties of a certain type, it is considered to be of that type.

This approach allows for more flexibility in code. Developers can create new types that work with existing functions, as long as the structure matches.

Structural typing is different from nominal typing used in some other languages. In nominal typing, types must be explicitly declared to be compatible.

This feature in TypeScript makes the language more adaptable and easier to use in many situations. It allows for duck typing, where the focus is on what an object can do rather than its specific type name.

TypeScript Fundamentals

TypeScript builds on JavaScript by adding static typing and other features. It helps catch errors early and improves code quality.

Key Features of TypeScript

TypeScript offers static typing, allowing developers to specify variable types. This catches errors at compile-time rather than runtime.

It supports object-oriented programming with classes, interfaces, and modules. These features make code more organized and reusable.

TypeScript includes advanced type checking. Developers can use union types, intersection types, and generics for flexible and robust code.

The language is compatible with existing JavaScript. TypeScript compiles to plain JavaScript, so it works in any environment that runs JS.

Benefits of Using TypeScript for Development

TypeScript improves code quality and reduces bugs. The static typing system catches errors early in the development process.

It enhances code readability and maintainability. Type annotations make code self-documenting and easier to understand.

TypeScript boosts developer productivity. Its intelligent code completion and refactoring tools speed up coding tasks.

The language scales well for large projects. Its modules and namespaces help organize complex codebases effectively.

TypeScript has strong tool support. Many popular IDEs and text editors provide excellent TypeScript integration.

It smooths the learning curve for developers new to static typing. TypeScript allows gradual adoption, letting teams migrate bit by bit.

Type System

TypeScript’s type system is a core feature. It lets developers specify types for variables, function parameters, and return values. This catches errors at compile-time instead of runtime.

Basic types include:

  • number
  • string
  • boolean
  • any
  • void

TypeScript also supports more complex types:

  • Arrays
  • Tuples
  • Enums
  • Objects

The compiler checks these types and flags errors if used incorrectly. This makes code more robust and easier to maintain.

Type inference is another useful feature. It lets TypeScript guess types when not specified. This reduces the need for explicit type annotations.

Interfaces and Type Aliases

Interfaces define object shapes in TypeScript. They specify the structure an object should have. This includes property names and their types.

Example:

interface Person {
  name: string;
  age: number;
}

Interfaces can be extended to create more specific types. They’re useful for defining contracts between different parts of code.

Type aliases create new names for types. They can represent simple or complex types. Type aliases are often used with union types.

Example:

type StringOrNumber = string | number;

Both interfaces and type aliases improve code readability and reusability. They help create clear, self-documenting code structures.

Advanced TypeScript Concepts

TypeScript offers powerful features that go beyond basic JavaScript. These advanced concepts help developers write more robust and scalable code.

Generics

Generics allow developers to create reusable components that work with different types. They provide flexibility while maintaining type safety.

Here’s a simple generic function:

function identity<T>(arg: T): T {
    return arg;
}

This function can work with any type, preserving type information.

Generics are useful for creating flexible interfaces and classes. They help avoid code duplication and improve type inference.

Generic constraints limit the types that can be used with a generic. For example:

function getProperty<T, K extends keyof T>(obj: T, key: K) {
    return obj[key];
}

This function ensures the key exists in the object type.

Decorators

Decorators are special declarations that can modify classes, methods, properties, or parameters. They add metadata or change behavior without altering the original code.

To use decorators, enable the experimentalDecorators compiler option.

Class decorator example:

function sealed(constructor: Function) {
    Object.seal(constructor);
    Object.seal(constructor.prototype);
}

@sealed
class Greeter {
    greeting: string;
    constructor(message: string) {
        this.greeting = message;
    }
}

This decorator seals the class, preventing new properties from being added.

Method decorators can modify or replace method definitions. Property decorators can change property behavior or add metadata.

Decorators are often used in frameworks and libraries to add functionality without complex inheritance structures.

Modules and Namespaces

Modules and namespaces help organize code and prevent naming conflicts. Modules are the preferred way to structure code in modern TypeScript.

A module is a file that contains at least one import or export statement. For example:

// math.ts
export function add(x: number, y: number): number {
    return x + y;
}

// app.ts
import { add } from './math';
console.log(add(1, 2));

Modules have their own scope, avoiding global namespace pollution.

Namespaces are used for logical grouping of functionalities. They’re less common in modern TypeScript but can be useful in certain scenarios.

namespace Validation {
    export interface StringValidator {
        isAcceptable(s: string): boolean;
    }
}

Namespaces can be split across multiple files and used with /// <reference> syntax.

Conclusion

I hope these 101 TypeScript interview questions and answers have been helpful in preparing you for your next TypeScript developer interview. I tried to cover a diverse set of questions spanning TypeScript fundamentals, advanced concepts, and real-world scenarios to give you comprehensive preparation.

Practicing these questions should build your confidence and sharpen your TypeScript skills. You’ll be ready to ace those commonly asked TypeScript interview questions and impress potential employers with your knowledge. Review the code examples until you thoroughly understand the concepts.

Remember, interviews are also about clearly communicating your thought process. So take your time, think through the problems logically, and explain your approach. With focused preparation using resources like these questions, you’ll be well on your way to landing that TypeScript developer job. I wish you the best of luck!