Top TypeScript Interview Questions (With Answers)

Introduction

TypeScript has become a most important part of modern web development, especially in large-scale applications where type safety and scalability are critical. As a JavaScript developer, TypeScript brings static typing, improved developer tooling, and better code maintainability - making it a favorite among companies and developers alike.

If you're preparing for a frontend or full-stack developer interview, having a strong grip of TypeScript is must. Interviewers are increasingly assessing not just your theoretical knowledge of TypeScript, but also your ability to solve real-world problems using its features.

In this guide, we've compiled the most commonly asked TypeScript interview questions, categorized by difficulty - from beginner to advanced. Whether you're just starting out or aiming for a senior-level role, these questions will help you review concepts, strengthen your understanding, and confidently tackle your next interview.

Basic TypeScript Interview Questions

These typescript interview questions are aimed at developers who have hands-on experience with TypeScript and are familiar with more advanced features. These are the most asked typescript interview questions.

1. What is TypeScript and how is it different from JavaScript?

TypeScript is an open-source programming language developed by Microsoft. It is a superset of JavaScript that adds static typing, interfaces, and other features to help catch errors during development.

The main difference between TypeScript vs JavaScript lies in static typing and type safety.

Example:

JavaScript:

1function greet(name) {
2return "Hello, " + name;
3}

TypeScript:

1function greet(name: string): string {
2return "Hello, " + name;
3}

In the TypeScript version, we've added a type annotation to ensure name is always a string and the function returns a string. This helps catch errors early.

Learn more about typescript vs javascript

2. What are the Key Features of TypeScript?

TypeScript offers several powerful features that make it a preferred choice for building large-scale and maintainable applications. Below are some of its key features:

• Static Type Checking
• Type Inference
• Interfaces and Types
• Classes and Object-Oriented Programming
• Enums
• Generics
• Compatibility with JavaScript

3. What is Type Inference in TypeScript?

TypeScript can automatically infer the type of a variable even if you don’t explicitly declare it.

1let name = "DevWhirl"; // inferred as string
2let age = 18;         // inferred as number

4. What are the basic types in TypeScript?

TypeScript provides several built-in types:

• string
• number
• boolean
• null
• undefined
• any
• void
• unknown
• never

1let username: string = "Depp";
2let isActive: boolean = true;
3let score: number = 100;

5. What is the difference between interface and type in TypeScript?

Both interface and type in TypeScript are used to define types, but they have differences in their usage and capabilities. While both allow you to define the shape of objects, there are some key distinctions that determine when you should use one over the other.

• interface can be extended using extends, and is preferred for defining contracts.

  .ts

  1interface Person {
  2name: string;
  3}
  4
  5interface Employee extends Person {
  6position: string;
  7}
  8
  9const employee: Employee = {
  10name: "Depp",
  11position: "Developer"
  12};

• type is more flexible and can use unions and intersections.

  .ts

  1type Person = {
  2name: string;
  3};
  4
  5type Employee = Person & {
  6position: string;
  7};
  8
  9const employee: Employee = {
  10name: "Depp",
  11position: "Developer"
  12};    

6. What are Enums in TypeScript?

Enums in TypeScript are a way to define a set of named constants, making your code more readable and maintainable. They can be used to represent values like days of the week, status codes, or directions.

Enums improve code readability and make the code more maintainable by giving meaningful names to numeric or string values.

Example:

1enum Direction {
2Up = 1,
3Down,
4Left,
5Right
6}
7
8let move: Direction = Direction.Up;
9console.log(move);  // Output: 1

7. What is the any type in TypeScript?

any type in typescript disables all type-checking for a variable. Use it cautiously.

1let data: any = "DevWhirl";
2data = 123; // No error

8. How does TypeScript handle optional properties?

In TypeScript, optional properties are defined using a question mark (?). This allows properties to be omitted when creating an object.

1interface User {
2name: string;
3age?: number;  // Optional
4}
5
6let user1: User = { name: "Depp" };
7let user2: User = { name: "joe", age: 25 };

9. List the Advantages of TypeScript

• Static typing helps catch errors early.
• Better code completion and IntelliSense in editors.
• Improves code readability and maintainability.
• Type system is optional and can be gradually adopted.
• Supports advanced features like interfaces and generics.
• Detects bugs at compile time before running the code.
• Supports latest JavaScript features with backward compatibility.
• Works well with popular frameworks and libraries.
• Large community and backed by Microsoft.
• Makes code refactoring easier and safer.

10. Define static typing?

Static typing means that variable types are known and checked at compile time, before the code is run. In statically typed languages like TypeScript, you declare or infer types, and the compiler ensures type correctness.

11. Can TypeScript be used for the backend?

Yes, TypeScript can be used for backend development, and it has become increasingly popular in modern backend projects.

TypeScript works seamlessly with Node.js, which is the most common runtime for building JavaScript-based backends. Popular frameworks like Express.js, NestJS, and Fastify offer excellent TypeScript support, enabling developers to build fully typed APIs and services with better tooling and maintainability.

Advanced TypeScript Interview Questions?

These typescript interview questions go deep into more complex features, testing your understanding of advanced concepts and real-world application. These are the most asked advanced typescript interview questions.

12. What are Utility Types in TypeScript?

Utility types in TypeScript are built-in types that help with common transformations and operations on other types. They simplify type manipulation and improve the reusability and flexibility of your code. TypeScript provides several utility types to make it easier to perform tasks like making types optional, readonly, or excluding certain properties.

Common Utility Types in TypeScript:

• Partial<T>: Makes all properties of T optional.
• Required<T>: Makes all properties of T required.
• Readonly<T>: Makes all properties of T read-only.
• Pick<T, K>: Selects specific properties from T.
• Omit<T, K>: Excludes specific properties from T.

1interface User {
2name: string;
3age: number;
4}
5
6const partialUser: Partial<User> = { name: "Depp" }; // 'age' is optional
7const readonlyUser: Readonly<User> = { name: "joe", age: 25 };
8// readonlyUser.name = "Depp"; // Error

13. What is Declaration Merging in TypeScript?

Declaration Merging in TypeScript is a powerful feature where multiple declarations with the same name are automatically merged by the compiler into a single definition. This allows you to extend existing types like interfaces, functions, namespaces, and enums without modifying the original code.

1interface User {
2name: string;
3}
4
5interface User {
6age: number;
7}
8
9const user: User = { name: "Depp", age: 25 }; // Merged interface

14. How does TypeScript handle Conditional Types?

Conditional types allow you to define types based on a condition. They follow the syntax: T extends U ? X : Y, which evaluates to X if T extends U, otherwise it evaluates to Y.

1type IsString<T> = T extends string ? "Yes" : "No";
2
3type Test1 = IsString<string>; // "Yes"
4type Test2 = IsString<number>; // "No"

15. What are the keyof, typeof, and infer keywords in TypeScript?

• keyof: Extracts the keys of a type as a union of string literals.

  .ts

  1type User = { name: string; age: number };
  2type UserKeys = keyof User; // "name" | "age"

• keyof: The typeof operator returns the type of a variable or expression. It's commonly used in combination with type annotations.

  .ts

  1let user = {
  2name: "Depp",
  3age: 18
  4};
  5
  6type User = typeof user;  // Same as { name: string; age: number }

• infer: The infer keyword is used in conditional types to extract or infer a type within a generic.

  .ts

  1type GetReturnType<T> = T extends (...args: any[]) => infer R ? R : never;
  2
  3type Result = GetReturnType<() => string>;  // string

16. What is the purpose of the never type in TypeScript?

The never type represents values that never occur. It's typically used in functions that throw exceptions or never return (e.g., infinite loops or error handling).

1function throwError(message: string): never {
2throw new Error(message);
3}

How to Prepare for a TypeScript Interview

Preparing for a TypeScript interview questions can be quite challenging, but breaking it down step-by-step can really help. Start with the basics - make sure you really understand types, interfaces, enums, classes, and generics. It's important to not just memorize, but to get comfortable using them. Then, try writing actual code. Solve problems on sites like LeetCode or HackerRank using TypeScript to get familiar with the syntax and debugging. Since many roles involve frameworks like React, Angular, or Node.js, spend some time learning how TypeScript fits into those ecosystems.

View ReactJs Interview Questions

Frequently Asked Questions

• Q: How can you use TypeScript with React?

  A: You can type React components by defining props and state with interfaces or types. TypeScript also supports typing hooks like useState and useContext for safer and clearer code.

• Q: Can TypeScript catch all runtime errors?

  A: No, TypeScript performs static type checking at compile time and cannot catch errors that occur at runtime due to logic or external factors.

• Q: How does TypeScript improve JavaScript development?

  A: TypeScript adds static typing, improved tooling, early error detection, and better code maintainability, making it easier to write and scale large JavaScript applications.

Conclusion

Preparing for a TypeScript interview questions doesn't have to be Tough. By understanding the core concepts, practicing coding regularly, and familiarizing yourself with common typescript interview questions, you can build the confidence needed to succeed. Remember to focus not just on memorizing answers, but on truly grasping how TypeScript works and why it's such a powerful tool for building scalable, maintainable applications. Keep practicing, stay curious, and you'll be well on your way to acing your next TypeScript interview!

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