Top TypeScript Interview Questions to Ask in 2025

TL;DR: Key Takeaways

• Focus on Practicality: Move beyond textbook definitions. Ask candidates to solve real-world problems using generics, utility types, and async patterns.
• Prioritize Type Safety: Questions about any vs. unknown, type guards, and interfaces reveal a candidate’s commitment to preventing runtime errors.
• Assess Architectural Thinking: Use questions about access modifiers and decorators to gauge if a candidate can design scalable, maintainable systems.
• Connect to Business Impact: A strong candidate can explain how TypeScript features like static typing and utility types directly reduce bugs, improve developer velocity, and lower maintenance costs.
• Actionable Next Step: Use our hiring checklist below to create a scorecard and a contextual live-coding challenge for your next TypeScript interview.

Who This Guide Is For

This guide is for CTOs, Engineering Managers, and Staff Engineers who need to hire TypeScript developers capable of building reliable, enterprise-grade applications. If you're responsible for a product's technical quality and need to ensure your interview process accurately identifies top talent—not just candidates who memorized definitions—this framework is for you.

Hiring engineers with superficial TypeScript knowledge introduces subtle bugs, slows development velocity, and accumulates technical debt, directly threatening your product roadmap and budget. A rigorous interview process that digs deeper than basic syntax is your most effective defense against these risks. This guide provides a structured, actionable framework of typescript interview questions, complete with model answers and evaluation criteria, designed to help you identify engineers capable of building reliable systems.

To get the most out of these questions, a well-structured interview process is key. For more on structuring the assessment itself, you can explore advanced online interview techniques that help ensure a robust and fair evaluation of every candidate. This article focuses specifically on the content of the interview, giving you the precise questions needed to differentiate a good TypeScript developer from a great one.

1. What is TypeScript and how does it differ from JavaScript?

This is one of the most fundamental typescript interview questions an interviewer will ask. It’s designed to quickly gauge a candidate's foundational knowledge and their understanding of why TypeScript exists. A strong answer demonstrates they grasp its core value proposition: bringing static type-checking to JavaScript to build more robust, scalable, and maintainable applications.

TypeScript is an open-source language developed by Microsoft that acts as a strict syntactical superset of JavaScript. This means any valid JavaScript code is also valid TypeScript code. The primary difference is that TypeScript adds static types, which allow you to define the types of variables, function parameters, and return values during development. This process, known as type-checking, happens at compile time before the code is executed.

Core Differences Explained

The key distinction lies in when errors are caught. JavaScript is dynamically typed, meaning type errors are only discovered at runtime, often after the code has been deployed. TypeScript, being statically typed, catches these errors in your code editor or during the build process, preventing entire classes of bugs from ever reaching production.

• JavaScript (Dynamic Typing):

  function add(a, b) {return a + b;}// Runtime error: "1" + 2 results in "12"console.log(add("1", 2));

• TypeScript (Static Typing):

  function add(a: number, b: number): number {return a + b;}// Compile-time error: Argument of type 'string' is not assignable to parameter of type 'number'.console.log(add("1", 2));

Why It Matters for Engineering Teams

Adopting TypeScript provides significant business value, especially for large-scale applications and growing teams. Companies like Microsoft (Office 365), Google (Angular), and Slack use it to improve code quality and developer productivity. The benefits include enhanced autocompletion, easier refactoring, and clearer code documentation, which are crucial for managing complex systems. Understanding these benefits is essential for many modern jobs in software engineering.

2. What are Interfaces and how do you use them?

This is another foundational entry in our list of typescript interview questions that tests understanding of TypeScript's structural typing system. An interviewer asks this to see if you can define and enforce consistent object shapes, a core practice for writing scalable and self-documenting code. A good answer shows you know how to create contracts for objects, classes, and functions.

In TypeScript, an interface is a powerful way of defining a contract that data structures must adhere to. It specifies the expected properties and methods an object should have, without providing an implementation. This is fundamental to TypeScript's principle of "duck typing" or structural typing, where if an object has the same shape as an interface, it is considered to be of that type. This allows for flexible yet predictable code.

Core Usage and Implementation

Interfaces are used to create explicit contracts within your code, ensuring that different parts of your application communicate correctly. They are particularly useful for defining the shape of props in a React component, structuring API responses, or ensuring consistency in database models. They act as a blueprint for objects.

• Example: Defining an API Contract

  interface UserProfile {readonly id: number;   // Cannot be changed after creationname: string;email: string;profileUrl?: string; // Optional property}function displayUser(user: UserProfile): void {console.log(`User: ${user.name}, Email: ${user.email}`);}// This object is valid because it matches the UserProfile interface shapeconst myUser = { id: 1, name: "Jane Doe", email: "jane.doe@example.com" };displayUser(myUser);

Why They Matter for Engineering Teams

Interfaces are a critical tool for team collaboration and maintaining large codebases. By defining clear contracts for public-facing APIs or component libraries, teams can work in parallel with confidence. For example, a backend team can provide an interface for an API endpoint, allowing a frontend team to build against that contract before the API is even live. This reduces integration friction and catches errors at compile-time, not runtime, directly improving developer productivity and code quality.

3. Explain Generics and provide practical use cases

This is a classic typescript interview questions that separates mid-level from junior developers. It tests your ability to think abstractly about code reuse and type safety. A strong answer demonstrates you can build flexible, scalable components that aren't tied to a single data type, a critical skill for creating libraries, APIs, and complex frontend states.

Generics in TypeScript allow you to write functions, classes, or interfaces that can work over a variety of types rather than a single one. This is achieved by using a type parameter, conventionally denoted by T, which acts as a placeholder for a specific type that will be provided when the code is used. This enables you to create reusable components while preserving type information and catching errors at compile time.

Core Concept Explained

The primary value of generics is creating a direct link between the type of an input and the type of an output. Without generics, you would either have to use the any type, which sacrifices type safety, or write duplicate code for each specific type. Generics solve this by allowing the compiler to enforce type consistency dynamically based on usage.

• Without Generics (loses type safety):

  function getFirstElement(arr: any[]): any {return arr[0];}const firstNum = getFirstElement([1, 2, 3]); // firstNum is 'any'const firstStr = getFirstElement(["a", "b"]); // firstStr is 'any'

• With Generics (maintains type safety):

  function getFirstElement<T>(arr: T[]): T | undefined {return arr[0];}// Type is inferred: 'firstNum' is of type 'number'const firstNum = getFirstElement([1, 2, 3]);// Type is inferred: 'firstStr' is of type 'string'const firstStr = getFirstElement(["a", "b"]);

Why It Matters for Engineering Teams

Generics are fundamental to building robust and maintainable systems. They are heavily used in libraries like React (e.g., useState<T>) and backend frameworks for creating type-safe data access layers (e.g., a generic repository pattern). By using generics, teams can reduce boilerplate code, prevent common type-related bugs, and create more predictable APIs. Mastering them is one of the key skills required for a software engineer working on modern, large-scale applications.

4. What is the difference between 'any', 'unknown', and 'never' types?

This is a classic intermediate-level entry in any list of typescript interview questions. It tests your understanding of TypeScript's type system beyond the basic primitives. An interviewer uses this to see if you can write type-safe code, especially when dealing with dynamic data, external APIs, or complex logic. A strong answer demonstrates your ability to balance flexibility with safety.

These three special types handle scenarios where a value's type isn't known or doesn't exist. any completely opts out of type-checking, unknown is a type-safe alternative that forces you to perform checks before using the value, and never represents a value that should never occur. Understanding the distinction is key to maintaining a robust and error-free codebase.

Core Differences Explained

The fundamental difference is the level of type safety each provides. any is the least safe, effectively reverting a variable to behave like plain JavaScript. unknown is much safer because it requires you to narrow down the type before performing any operations. never is used for control flow analysis to represent impossible states.

• any (The Escape Hatch):
  Using any disables all type-checking for that variable. You can call any method or access any property on it without a compile-time error, which defers potential issues to runtime. It's often used for migrating legacy JavaScript code.

  let data: any = "This could be anything";console.log(data.nonExistentMethod()); // No compile-time error, but will crash at runtime.

• unknown (The Type-Safe Alternative):
  A variable of type unknown can hold any value, but you cannot operate on it until you perform a type check (e.g., using typeof or instanceof). This is ideal for values from external sources like API responses or user input.

  let value: unknown = "This is a string";if (typeof value === "string") {console.log(value.toUpperCase()); // OK: TypeScript now knows 'value' is a string.}

• never (The Impossible State):
  The never type represents values that can never happen. It's used for functions that always throw an error or have an infinite loop. It’s also invaluable for ensuring exhaustive checks in switch statements, preventing bugs when new cases are added.

  function exhaustiveCheck(param: never): never {throw new Error("This function should never be called.");}

Why It Matters for Engineering Teams

Choosing the correct type has a direct impact on code quality and maintainability. While any can speed up initial development or migration, it introduces "type debt" that can lead to runtime errors. Promoting the use of unknown enforces safer coding practices across the team. Using never for exhaustive checks makes the compiler an ally in ensuring all logical paths are handled, which is critical for building reliable systems in a large-scale application.

5. What are Union Types and Type Guards?

This is a very common type of typescript interview questions that moves beyond basic syntax to test your understanding of type flexibility and safety. The question assesses your ability to model real-world scenarios where a value can be one of several types, a frequent occurrence in APIs and state management. A strong answer shows you can not only define flexible types but also safely narrow them down for consumption.

A Union Type is a powerful feature that allows a variable, parameter, or property to hold a value of one of several distinct types. It’s declared using the pipe (|) symbol between the types. Type Guards are expressions or functions that perform a runtime check to guarantee the type of a variable within a specific block of code. This process, known as type narrowing, is essential for working safely with union types, as TypeScript’s compiler uses these guards to infer a more specific type.

Core Concepts Explained

The key benefit is combining flexibility with type safety. You can design functions or components that accept multiple input types without sacrificing the compile-time checks that prevent runtime errors. Type guards are the mechanism that makes this combination practical and safe.

• Union Type (Flexibility): A function can accept either a string or a number.

  function logIdentifier(id: string | number) {// TypeScript Error: Property 'toUpperCase' does not exist on type 'string | number'.// console.log(id.toUpperCase());}

• Type Guard (Safety): Using typeof to narrow the union.

  function logIdentifier(id: string | number) {if (typeof id === "string") {// Inside this block, TS knows 'id' is a string.console.log(id.toUpperCase());} else {// Inside this block, TS knows 'id' is a number.console.log(id.toFixed(2));}}

  Common built-in type guards include typeof for primitives, instanceof for classes, and the in operator for checking property existence.

Why It Matters for Engineering Teams

Union types and type guards are foundational for building resilient systems. In modern application development, such as with React components receiving varied props or handling API responses that can be either a success or error object, unions are indispensable. For instance, a common pattern is the discriminated union, where objects in a union share a common literal property. This allows for highly effective and readable type narrowing using a switch statement, ensuring all possible cases are handled—a concept known as exhaustiveness checking. This pattern significantly reduces bugs in complex state machines and data flows.

6. Explain Access Modifiers (public, private, protected) and their use cases

This is a classic Object-Oriented Programming (OOP) question adapted for TypeScript, making it a staple in typescript interview questions. The interviewer wants to verify your understanding of encapsulation, a core OOP principle for creating secure and maintainable code. A strong answer shows you can design robust class structures that prevent unintended data manipulation.

Access modifiers in TypeScript control the visibility and accessibility of class members (properties and methods). By explicitly defining who can access what, you enforce boundaries and create a clear, intentional API for your classes. TypeScript provides three main access modifiers: public, private, and protected.

Core Modifiers Explained

The key difference between these modifiers is the scope from which class members can be accessed. Understanding these scopes is crucial for building scalable and encapsulated components, especially in large codebases where multiple developers collaborate.

• public: This is the default modifier if none is specified. Public members are accessible from anywhere, both inside and outside the class.

  class BankAccount {public ownerName: string; // Accessible everywhereconstructor(name: string) {this.ownerName = name;}}const account = new BankAccount("Alex");console.log(account.ownerName); // Works

• private: Members marked as private are only accessible from within the class they are defined in. This is the strictest level of encapsulation, ideal for internal state or helper methods.

  class BankAccount {private _balance: number = 0; // Only accessible inside BankAccountdeposit(amount: number): void {this._balance += amount; // Works}}const account = new BankAccount();// account._balance; // Compile-time error: Property '_balance' is private.

• protected: Protected members are accessible within the defining class and by any subclasses that inherit from it. This is useful for creating base classes with internal logic that derived classes can use.

  abstract class Vehicle {protected speed: number = 0;}class Car extends Vehicle {public accelerate() {this.speed += 10; // Can access protected 'speed'}}

Why It Matters for Engineering Teams

Properly using access modifiers is fundamental to building reliable systems. In a fintech application, a private balance prevents other parts of the system from arbitrarily changing an account's value. In a complex UI library, protected methods in a base component allow for extension while hiding implementation details. This discipline leads to fewer bugs, easier refactoring, and a more predictable system, directly impacting developer productivity and code quality.

7. What are Enums and when should they be used?

This is a classic typescript interview questions that tests your knowledge of TypeScript's special features beyond basic types. The interviewer wants to see if you understand how to create a fixed set of named constants and, more importantly, when this pattern is appropriate versus more modern alternatives like union types. A good answer demonstrates practical wisdom about trade-offs in maintainability and performance.

An enum (short for enumeration) in TypeScript is a feature that allows you to define a set of named constants, making your code more readable and less prone to errors from magic numbers or strings. By default, enums are number-based, with members auto-incrementing from 0, but they can also be string-based. This helps create a type-safe way to work with a finite set of values, like user roles or application states.

Core Use Cases Explained

The primary value of enums is to give meaningful names to a set of related constants. This improves code clarity and self-documentation. For example, instead of using 0, 1, and 2 to represent user roles, you can define an enum that clearly states the purpose of each value.

• Numeric Enum (Default):

  enum UserRole {Admin, // 0User,  // 1Guest  // 2}function checkPermissions(role: UserRole) {if (role === UserRole.Admin) {// ... logic for admin}}

• String Enum (Recommended for Debugging):

  enum LogLevel {Debug = "DEBUG",Info = "INFO",Warn = "WARN",Error = "ERROR"}// The log output "INFO" is more readable than a number like 1console.log(`Processing data with level: ${LogLevel.Info}`);

When It Matters for Engineering Teams

Using enums is valuable for defining domain-specific constants that are shared across a codebase, such as status codes, configuration flags, or event types. For performance-critical code, const enums are a powerful optimization. They are completely erased during compilation, and their values are inlined directly into the JavaScript output, resulting in no runtime overhead. However, be cautious as this can have implications for separate compilation. Many modern TypeScript projects, especially in companies like Airbnb and across the open-source community, often prefer using string literal union types (type Role = 'Admin' | 'User' | 'Guest') as a more lightweight and JavaScript-native alternative.

8. Explain Decorators and their applications

This is a more advanced topic among typescript interview questions, often reserved for mid-level or senior roles. It evaluates your knowledge of metaprogramming concepts and your familiarity with popular frameworks like Angular or NestJS. A strong answer shows you understand how decorators reduce boilerplate and enable powerful, declarative patterns.

Decorators are a special kind of declaration that can be attached to a class, method, accessor, property, or parameter. They are functions that are prefixed with an @ symbol and are executed at design time. Their purpose is to modify or observe the definitions of their targets, adding metadata or altering their behavior without directly changing their source code. They are currently an experimental feature but are widely used.

Core Applications Explained

Decorators enable aspect-oriented programming (AOP) patterns, where cross-cutting concerns like logging, validation, or dependency injection are separated from business logic. This makes code cleaner and more modular. To use them, you must enable the experimentalDecorators flag in your tsconfig.json file.

• Frameworks (Angular & NestJS): Decorators are fundamental. Angular uses @Component to define a component's metadata, while NestJS uses @Controller, @Get, and @Injectable to define routing and dependency injection.

  // NestJS Example: A simple controllerimport { Controller, Get } from '@nestjs/common';@Controller('users')export class UsersController {@Get()findAll(): string {return 'This action returns all users';}}

• Validation: Libraries like class-validator use decorators to define validation rules directly on model properties.

  import { IsString, IsEmail } from 'class-validator';class User {@IsString()name: string;@IsEmail()email: string;}

Why They Matter for Engineering Teams

Decorators significantly improve developer experience and code readability by abstracting complex or repetitive logic into reusable functions. In large-scale applications managed by teams at companies like Google (Angular) and in the growing NestJS ecosystem, decorators are key to maintaining a clean, scalable architecture. They allow developers to express intent declaratively, which reduces cognitive load and makes the codebase easier to navigate and maintain.

9. What are Utility Types and how do they improve code reusability?

This question is a staple in many typescript interview questions for mid-level and senior roles. It evaluates your ability to write clean, reusable, and type-safe code without reinventing the wheel. A great answer demonstrates fluency with TypeScript's built-in type manipulation tools, showing you can model complex data transformations efficiently and maintainably.

Utility Types are built-in generic types that provide common type transformations. They take one or more existing types as input and return a new, modified type. This allows you to create new types based on existing ones without manually defining them, which is a core principle of Don't Repeat Yourself (DRY) in programming. They are essential for creating flexible and reusable type definitions, especially when dealing with API contracts or component props.

Core Utility Types Explained

The power of Utility Types lies in their ability to adapt existing types for specific scenarios, reducing boilerplate and preventing type mismatches. JavaScript is dynamically typed, so these concepts don't have a direct equivalent; all transformations happen at the type level before compilation.

• Scenario: You have a User type but need to create a function that updates only some of its properties.

  interface User {id: number;name: string;email: string;isActive: boolean;}// Creates a new type where all properties of User are optional.// updateUser can now safely accept a partial User object.function updateUser(id: number, data: Partial<User>): void {// ... logic to update user}updateUser(1, { name: "Jane Doe" }); // Valid

• Pick<User, 'id' | 'email'>: Creates a type with only the id and email properties from User.
• Omit<User, 'id'>: Creates a type with all properties from User except id.
• Readonly<User>: Makes all properties of the User type read-only, preventing modification.
• Record<string, number>: Creates a type for an object with string keys and number values.

Why It Matters for Engineering Teams

Mastery of Utility Types directly translates to faster development and more robust systems. For teams at companies like Airbnb and Asana, they are indispensable for managing complex state and API layers. Using Pick and Omit to shape data for public-facing APIs ensures you don't accidentally expose sensitive internal fields. Similarly, Partial is invaluable for handling form state in frameworks like React, while Readonly helps enforce immutability patterns, leading to more predictable state management and fewer bugs.

10. How do you handle Async/Await type safety and error handling in TypeScript?

This is a critical mid-to-senior level question among typescript interview questions, as it tests your ability to manage asynchronous operations, a core part of modern web development. Interviewers want to see that you can write resilient, predictable code that fetches data or performs I/O tasks without introducing runtime errors or unhandled promise rejections. A strong answer shows mastery of Promise typing and robust error-handling patterns.

TypeScript enhances asynchronous JavaScript by allowing you to type the resolved value of a Promise. When using async/await, you must explicitly define the return type of an async function as Promise<T>, where T is the type of the data the promise will resolve with. This enables the TypeScript compiler to catch type mismatches before your code ever runs, preventing bugs related to incorrect data shapes from API responses or database queries.

Core Differences Explained

Properly handling async/await involves two key parts: typing the promise resolution and catching potential errors. In vanilla JavaScript, you might forget to handle a failed network request, leading to a crash. In TypeScript, you combine explicit return types with try...catch blocks to create type-safe and robust asynchronous logic.

• Unsafe Async Operation:

  // The return type is inferred as Promise<any>, offering no type safety.async function fetchUser(id: number) {const response = await fetch(`https://api.example.com/users/${id}`);const user = await response.json();return user;}

• Type-Safe and Robust Async Operation:

  interface User {id: number;name: string;}// Explicitly types the resolved value and handles potential errors.async function fetchUser(id: number): Promise<User | null> {try {const response = await fetch(`https://api.example.com/users/${id}`);if (!response.ok) {throw new Error('Network response was not ok');}const user: User = await response.json();return user;} catch (error) {console.error("Failed to fetch user:", error);return null; // Return a predictable value on failure.}}

Why It Matters for Engineering Teams

For any application that interacts with APIs, databases, or the file system, robust async handling is non-negotiable. Teams at companies like Netflix and Airbnb rely on these patterns to build resilient microservices and user interfaces. Correctly typing promises ensures that data flowing through an application is predictable, which simplifies state management and reduces bugs. Furthermore, systematic error handling prevents silent failures and improves the user experience, which is a key concern for any team where software engineers are in demand.

TypeScript Interview Question Checklist & Scorecard

Use this table as a quick reference during interviews to evaluate a candidate's depth of knowledge across key TypeScript concepts.

What To Do Next

1. Customize Your Scorecard: Use the checklist above to create a hiring scorecard. Weight the topics that are most critical for the specific role you're hiring for.
2. Prepare a Live-Coding Challenge: Design a small, practical problem that requires the candidate to use 2–3 of these concepts together. For example, ask them to fetch and display data, which will test their knowledge of async, Promise<T>, interfaces, and type guards.
3. Ask "Why?": During the interview, always follow up with "Why did you choose that approach?" or "What are the trade-offs?" This separates candidates who know syntax from those who have deep architectural understanding. For instance, a great candidate will discuss when an interface is better than a type and vice-versa, citing specific examples of declaration merging or union type limitations.

Understanding the landscape of available remote TypeScript jobs is also crucial for benchmarking your roles and reaching the right candidates. By combining a deep question bank with a robust evaluation framework, you transform your interview from a simple knowledge check into a powerful predictor of on-the-job performance.

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