TypeScript Interview Questions & Answers

What are TypeScript's primitive types?

any vs unknown vs never?

What is a union type and when do you use it?

Intersection types — what are they for?

type vs interface for object shapes?

What are literal types?

Explain void vs undefined as return types.

How do tuples differ from arrays in TS?

What is declaration merging for interfaces?

How do you extend interfaces?

Optional vs readonly properties?

Index signatures — uses and pitfalls?

How do function types appear in interfaces?

Interface for API response modeling tips?

When do interfaces become an anti-pattern?

What problem do generics solve?

Explain generic constraints (extends).

Default type parameters — when useful?

keyof and indexed access with generics?

Generic variance intuition (in/out)?

Why inference sometimes fails with generics?

Generics vs overloads — how to choose?

Design a type-safe event emitter with generics.

What does Partial<T> do?

Pick vs Omit?

Record<K, V> use cases?

Required and Readonly?

ReturnType and Parameters?

NonNullable and Awaited?

When should you write custom utilities instead of built-ins?

What is control-flow narrowing?

How does typeof narrowing work and its limits?

Discriminated unions — pattern?

Truthy narrowing pitfalls?

in operator narrowing?

Assertion functions (asserts)?

Why does narrowing fail after assigning to a variable?

What is a user-defined type guard?

instanceof vs custom guards?

How do you type-guard arrays of unknowns?

Zod/io-ts vs hand-written guards?

Exhaustiveness checks with never?

Type predicates with generics?

Security note on type guards?

How do parameter properties work?

public/private/protected vs #private?

abstract classes vs interfaces?

What does implements do?

Class field initialization order pitfalls?

When are classes preferable to closures/factories?

Typing this in class methods/callbacks?

What are decorators in TypeScript?

legacy experimentalDecorators vs new decorators?

Common decorator use cases?

How do class decorators run conceptually?

Decorator metadata reflection caveats?

Testing code that relies on decorators?

When should a team avoid decorators?

What is a branded/nominal type pattern?

Template literal types — uses?

What are recursive types good for?

Satisfies operator — why use it?

const type parameters?

Polymorphic this types?

When is type-level programming too much?

What is a conditional type?

What is distributivity in conditional types?

infer keyword — what does it do?

Extract and Exclude implemented how?

How do you flatten Promise types with conditionals?

Debugging monstrous conditional types?

Library design: expose conditionals or simpler overloads?

What is a mapped type?

Key remapping with as — example use?

How do modifiers +, - work in mapped types?

Homomorphic mapped types — why special?

Build a DeepReadonly<T>?

Mapped types vs runtime mapping?

When mapped types hurt DX?

How does contextual typing work?

When should you annotate vs rely on inference?

Why does let x = [] become any[]?

as const assertion effects?

Return type inference with complex conditionals?

Inference across generic call chains?

Design APIs for good inference.

What does strict mode enable that's important?

Why is any contagious?

Safe patterns for JSON.parse?

Non-null assertion (!) risks?

Type assertion vs type conversion?

How do you migrate a JS codebase toward type safety?

Soundness vs productivity trade-offs?

How do you structure types in a large TS monorepo?

DTO vs domain model typing?

Dependency injection typing approaches?

How do path aliases affect architecture?

Public API surface for a TS library?

Error typing strategies?

Lead: set TypeScript engineering standards for a team.