TypeScript Interview Questions and Answers for 2025

The first time I used TypeScript in a real project, I thought it was just JavaScript with type annotations. Add some colons and angle brackets, make the compiler happy, and you're done. Then I ran into a bug that TypeScript would have caught immediately if I'd actually understood the type system. That's when I realized: TypeScript isn't just typed JavaScript. It's a different way of thinking about code.

This distinction comes up constantly in TypeScript interviews. Interviewers aren't just checking if you know the syntax. They want to know if you understand how to use types to catch bugs, document your code's intent, and make refactoring safer. Let's break down what that looks like at each experience level.

Junior Level - Types as Documentation

When companies hire junior TypeScript developers, they're usually hiring JavaScript developers who are learning TypeScript. The expectation isn't that you're a type system expert—it's that you understand how basic types make code more reliable and readable.

The fundamentals that matter:

Can you use basic type annotations? This means knowing how to type function parameters, return values, and variables. function greet(name: string): string is more self-documenting than function greet(name). Even without running the code, you know what type of argument it expects and what it returns.

Do you understand the difference between interface and type? For most use cases, they're interchangeable. But understanding when you'd use each shows you're thinking about code organization. Interfaces are great for object shapes that might be extended. Type aliases are more flexible for unions, intersections, and primitive types.

Can you work with basic generics? You don't need to write complex generic types, but you should understand Array<string> and how to use generic functions from libraries. When you see Promise<User>, you should understand that this is a promise that resolves to a User object.

A typical junior-level question:

"Define a TypeScript interface for a blog post object with title, content, author, and publish date. Then write a function that takes an array of these posts and returns only published posts."

This tests basic interface definitions, array typing, and function signatures. A good answer might look like:

interface BlogPost {
  title: string;
  content: string;
  author: string;
  publishDate: Date;
  isPublished: boolean;
}

function getPublishedPosts(posts: BlogPost[]): BlogPost[] {
  return posts.filter(post => post.isPublished);
}

What I'm looking for is clarity and correctness. Are your types accurate? Does your implementation match your types? Can you explain why types help here?

Mid-Level - Leveraging the Type System

Mid-level TypeScript interviews expect you to use the type system effectively, not just annotate code. You should understand how types can catch bugs before they happen and make refactoring safer.

What separates mid-level from junior:

Can you use union and intersection types effectively? Union types (string | number) represent values that could be one of several types. Intersection types (User & Timestamps) combine multiple types. Understanding these helps you model complex data accurately.

Do you understand type narrowing and type guards? When you check typeof value === 'string', TypeScript narrows the type within that block. Custom type guards using is let you create your own narrowing logic. This is crucial for handling different data shapes safely.

Can you work with utility types? Partial<T>, Pick<T, K>, Omit<T, K>, Record<K, V>—these built-in utilities save you from writing boilerplate types. More importantly, understanding them shows you think in terms of type transformations.

How do you handle async code in TypeScript? You should be comfortable with Promise<T> and async/await with proper typing. Understanding that an async function always returns a promise, even if you don't explicitly type it that way.

A realistic mid-level challenge:

"Create a type-safe API client function that handles different HTTP methods (GET, POST, PUT, DELETE) with appropriate request and response types."

This tests your ability to use generics, union types, and conditional logic in types. A strong answer might use function overloads or a generic function with conditional types to ensure POST/PUT require a body while GET/DELETE don't.

I look for candidates who use types to enforce correctness. If your types make it impossible to call the API incorrectly, that's excellent design. If types are just documentation that can be ignored, you're not getting the full benefit of TypeScript.

Senior Level - Advanced Type System Patterns

Senior TypeScript interviews dive into advanced type system features, type-level programming, and architectural decisions about how to structure type definitions across a large codebase.

What senior developers need to demonstrate:

Can you use mapped types and conditional types? Mapped types let you transform existing types: type Readonly<T> = { readonly [P in keyof T]: T[P] }. Conditional types enable type-level logic: type IsString<T> = T extends string ? true : false. These are powerful tools for creating flexible, reusable types.

Do you understand template literal types? These let you create precise string types: type HTTPHeader = \${'GET' | 'POST'}-${'json' | 'xml'}`creates a type with values like'GET-json'or'POST-xml'`. This is incredibly useful for type-safe APIs and configurations.

How do you handle complex type inference? TypeScript's type inference is powerful, but sometimes you need to guide it with generics, type assertions, or const assertions. Understanding when to let TypeScript infer and when to be explicit is a senior skill.

What's your approach to organizing types in large codebases? Do you colocate types with implementation or separate them? How do you handle shared types? How do you version API types? These aren't just technical questions—they're about maintainability and team workflow.

A senior-level architecture question:

"Design a type-safe event bus system where event names are strongly typed to their payload types, preventing listeners from receiving incorrectly typed data."

A strong solution might use a registry of event types, mapped types to create typed emit/on functions, and generic constraints to ensure type safety. You'd discuss trade-offs between flexibility and type safety, and how to make the developer experience smooth while maintaining guarantees.

The TypeScript Features That Always Come Up

Type inference vs explicit types: TypeScript can infer many types, but explicit types serve as documentation and catch mistakes. Finding the right balance is an art. Over-typing makes code verbose. Under-typing loses TypeScript's benefits.

any vs unknown: Using any disables type checking—it's an escape hatch that should be rare. unknown is safer: it represents any value, but you must narrow the type before using it. Understanding when each is appropriate shows maturity with the type system.

Non-null assertion operator: The ! operator tells TypeScript "I know this isn't null/undefined even though you think it might be." This is sometimes necessary, but overuse indicates you're fighting the type system instead of working with it.

Discriminated unions: This pattern uses a literal type field to narrow union types. It's incredibly useful for modeling state machines or different shapes of API responses. A solid understanding of discriminated unions indicates someone who thinks in TypeScript patterns.

Real-World TypeScript Challenges

Typing third-party libraries: Not every library has great TypeScript support. Can you write declaration files? Can you work with @types packages? Can you handle libraries with poor or missing types?

Migration from JavaScript: Many projects are partially TypeScript. Can you introduce types gradually? Do you understand // @ts-check and JSDoc typing for incremental migration? This is a common real-world scenario.

Build tool integration: TypeScript doesn't run natively in browsers or Node. Understanding how TypeScript fits into your build process—whether that's webpack, vite, or tsc directly—is practical knowledge that matters.

Type performance: In large codebases, complex types can slow down the compiler. Understanding when types are too complex and how to simplify them is an advanced skill.

Common Mistakes in TypeScript Interviews

Using any everywhere: This defeats the purpose of TypeScript. If you're constantly using any to make the compiler stop complaining, you're not really using TypeScript. Take the time to model types correctly.

Over-engineering types: Sometimes a simple type is better than a clever mapped type with conditionals. If your type is hard to understand, it might be too complex. Types are for humans as much as for the compiler.

Not understanding type vs runtime: TypeScript types are erased at runtime. You can't check if (typeof value === MyInterface) because interfaces don't exist at runtime. Confusing compile-time and runtime is a common mistake.

Fighting the type system: If you're constantly using type assertions and non-null assertions to make code compile, something is wrong. Either your types are incorrect, or your approach needs rethinking.

How Types Improve Real Development

This is what separates people who use TypeScript from people who understand its value.

Refactoring becomes safer: Change a function signature and TypeScript shows you everywhere that needs updating. Rename a property and the compiler finds every usage. This makes large-scale changes less scary.

Documentation that can't go stale: Comments can become outdated. Type signatures are verified by the compiler. If a function takes a User object, that's not just documentation—it's enforced.

Catching bugs at compile time: Typos in property names, passing wrong argument types, forgetting to handle null cases—TypeScript catches these before you run the code. This is especially valuable in larger codebases where manual testing can't cover everything.

Better IDE experience: Autocomplete, inline documentation, go-to-definition—these features are dramatically better with TypeScript. This isn't just convenience—it makes you more productive.

Preparing for TypeScript Interviews

Convert a JavaScript project to TypeScript: Take something you've built in JavaScript and add types. You'll encounter real challenges—dealing with third-party libraries, deciding how strict to be, handling edge cases. This experience is invaluable.

Read the TypeScript handbook: The official docs are excellent. They explain not just what features exist, but why they exist and when to use them. Understanding the reasoning behind features helps you use them effectively.

Study real TypeScript code: Look at popular TypeScript projects on GitHub. See how they structure types, handle complex scenarios, and organize type definitions. You'll learn patterns that work at scale.

Practice explaining type system concepts: Can you explain generics to someone who doesn't know TypeScript? Can you describe why type narrowing matters? Being able to articulate these concepts shows deep understanding.

Understand the trade-offs: TypeScript adds complexity and build steps. Not every project benefits equally. Understanding when TypeScript adds value and when it's overkill shows mature thinking.

The Mental Model That Matters

The best TypeScript developers don't just know the syntax—they think in types. When designing a function, they consider what types make invalid states unrepresentable. When modeling data, they use the type system to enforce business rules.

This mindset shows up in interviews. When given a problem, do you think about types from the start? Or do you write JavaScript and add types as an afterthought?

What Interviewers Actually Want to See

Type safety, not type acrobatics: Clever type-level programming is impressive, but clear, safe types that prevent bugs are more valuable. Show you can use types to make code more reliable, not just to show off type system knowledge.

Pragmatic decisions: Sometimes any is the right choice for a quick prototype. Sometimes strict mode is too strict for legacy code. Understanding when to bend the rules shows experience.

Communication about types: Can you explain why you chose certain types? Can you discuss trade-offs between different type approaches? Types are a way of communicating intent—being able to talk about them clearly is crucial.

Success at Each Level

Junior TypeScript developers should focus on basic type annotations, understanding primitive types, interfaces, and simple generics. Write types that make your code more self-documenting.

Mid-level developers need to understand union types, type guards, utility types, and how to use types to prevent entire classes of bugs. Show you can design type-safe APIs.

Senior developers should be comfortable with advanced type system features, able to design complex type systems, and capable of making architectural decisions about typing strategies in large codebases.

But at every level, the goal is the same: use types to make code more reliable, more maintainable, and easier to understand. That's what TypeScript is for, and that's what interviews are really testing.