TypeScript / Type System Interview Questions 2026

Your team wants to adopt TypeScript but has a 200K-line JavaScript codebase. What's the migration strategy?

You have a TypeScript generic function that takes an object and a key. It should return the value at that key in a type-safe way so callers get proper autocomplete. Write the function signature.

Explain the difference between 'unknown' and 'any' in TypeScript. When would using 'any' be a valid technical choice rather than a code smell?

You have a TypeScript function that accepts a union type of 10 different shapes. You use instanceof checks that work in tests but fail in production after webpack bundling. Why?

What is declaration merging in TypeScript and what problems can it solve?

Your TypeScript codebase has many 'as any' casts. A new developer says 'it's just to make the code compile.' What's the real cost?

Explain TypeScript's conditional types. Write one that extracts the return type of a Promise.

What is the difference between TypeScript's interface and type alias? When would you use each?

Explain TypeScript's structural typing. How does it differ from nominal typing and why does it sometimes cause unexpected behavior?

Explain the difference between a TypeScript interface and a type alias. Give a concrete case where only one of them can express what you need.

Your TypeScript code compiles fine but throws 'Cannot read property X of undefined' at runtime. How is this possible in a strictly typed TypeScript codebase?

What are TypeScript conditional types? Write a type that returns the element type of an array, or the type itself if not an array.

You're migrating a large JavaScript codebase to TypeScript. You have 500 files. What's your migration strategy to avoid a big-bang rewrite?

Explain the TypeScript utility types: Partial<T>, Required<T>, Pick<T,K>, Omit<T,K>, and Record<K,V>. Give a real use case for each.

You have a TypeScript codebase where a function returns string | undefined. Throughout the codebase, developers use the ! non-null assertion operator everywhere. Why is this a code smell?