--- title: "TypeScript Interview Questions (2026): By Level, With Model Answers" url: https://weworkworldwide.com/typescript-developer-interview-questions/ description: "TypeScript interview questions for junior, mid and senior developers — types vs interfaces, generics, narrowing and the type system — with answers and red flags." date: 2026-07-04T15:29:00+00:00 source: https://weworkworldwide.com/llms.txt --- # TypeScript Interview Questions (2026): By Level, With Model Answers How to use this TypeScript rewards people who understand that types are a design tool, not annotations to satisfy the compiler. These questions tell the two apart. Hiring a TypeScript developer is easy. Telling a real one from a convincing résumé is the hard part — and it’s most of what we do. These are grouped by level, because the same question that stretches a junior is a warm-up for a senior. ## Junior TypeScript interview questions 0–2 years Basic types and everyday usage. ### What problems does TypeScript solve over JavaScript? What a strong answer covers Static typing catches a class of bugs at compile time, improves editor tooling and refactoring, and documents intent; it compiles away to plain JS. Red flag Thinks TypeScript changes runtime behaviour. ### What is the difference between `interface` and `type`? What a strong answer covers Both describe object shapes; interfaces are open to declaration merging and extension, while `type` aliases can express unions, intersections and mapped types. Largely interchangeable for objects. Red flag Insists one is strictly better with no nuance. ### What are `any`, `unknown` and `never`? What a strong answer covers `any` opts out of checking, `unknown` is safe-any that must be narrowed before use, `never` represents values that can’t occur. Red flag Reaches for `any` as the default fix for type errors. ### What are union and intersection types? What a strong answer covers A union (`A | B`) is one of several types; an intersection (`A & B`) combines them. They know unions need narrowing before member access. Red flag Confuses which operator does which. ### What does the `?` optional modifier do? What a strong answer covers Marks a property or parameter as possibly `undefined`; the value must be checked before use under `strictNullChecks`. Red flag Ignores that optional means possibly undefined. ### What is type inference? What a strong answer covers The compiler deduces types from usage, so explicit annotations aren’t always needed; over-annotating adds noise. Red flag Annotates everything, including obvious inferred types, or annotates nothing. ### What is an enum, and are there alternatives? What a strong answer covers A named set of constants; union of string literals is often preferred for tree-shaking and simplicity. Red flag Unaware of literal-union alternatives. ### What does `readonly` do? What a strong answer covers Prevents reassignment of a property at compile time; it is not deep and disappears at runtime. Red flag Thinks `readonly` deep-freezes at runtime. ## Mid-level TypeScript interview questions 2–5 years Generics and narrowing. ### What are generics and why use them? What a strong answer covers Type parameters that let a function or type work over many types while preserving the relationship between inputs and outputs; the alternative is losing type safety with `any`. Red flag Uses `any` where a generic would keep type information. ### How does type narrowing work? What a strong answer covers Control-flow analysis narrows a union with `typeof`, `instanceof`, `in`, equality checks and discriminated unions, so member access is safe within a branch. Red flag Casts with `as` instead of narrowing properly. ### What is a discriminated union? What a strong answer covers A union of object types sharing a literal `kind`/`type` tag, letting the compiler narrow exhaustively in a switch. Pairs well with a `never` exhaustiveness check. Red flag Models variants with optional fields and lots of null checks instead. ### What do `keyof` and indexed access types do? What a strong answer covers `keyof T` yields the union of T’s keys; `T[K]` gets the type of a property, enabling type-safe generic accessors. Red flag Never uses them and hardcodes key strings. ### What is the difference between `as` and a type guard? What a strong answer covers `as` is an unchecked assertion that can lie to the compiler; a type guard actually narrows based on a runtime check. Red flag Sprinkles `as` to silence errors. ### What are utility types like `Partial`, `Pick`, `Omit`? What a strong answer covers Built-in mapped types that transform existing types — making properties optional, selecting or excluding keys — reducing duplication. Red flag Rewrites derived types by hand and lets them drift. ### How do you type an async function’s return? What a strong answer covers It returns a `Promise`; the resolved type is `T`. They know `await` unwraps it. Red flag Types the return as the resolved value without the Promise wrapper. ### What is structural typing? What a strong answer covers TypeScript compares types by shape, not by name, so any object with the required members is assignable. They understand duck typing implications. Red flag Assumes nominal typing like Java/C#. ## Senior TypeScript interview questions 5+ years Advanced types and config. ### What are conditional and mapped types? What a strong answer covers Types that branch (`T extends U ? X : Y`) or transform each property of a type; the basis for the utility types and powerful library APIs. Red flag Cannot read or write a mapped/conditional type. ### What do `infer` and template literal types enable? What a strong answer covers `infer` extracts a type within a conditional type (e.g. a promise’s resolved type); template literal types build string types compositionally. Red flag No exposure to type-level programming. ### How do you configure strictness and why? What a strong answer covers Enable `strict` (which includes `strictNullChecks`, `noImplicitAny`, etc.) to catch the most bugs; loosening it should be a deliberate, local decision. Red flag Ships with `strict` off to avoid fixing errors. ### How do you type third-party JS without types? What a strong answer covers Write or install declaration files (`.d.ts`), use module augmentation, and avoid blanket `any`. Red flag Casts everything from the library to `any`. ### What is declaration merging and when is it useful? What a strong answer covers The compiler merges multiple declarations of the same interface/namespace, useful for extending library types or global augmentation. Red flag Unaware it exists. ### How do generics with constraints work? What a strong answer covers `` restricts the type parameter so you can safely access members of `U` while staying generic. Red flag Writes unconstrained generics then casts inside. ### What is variance and why does it matter for function types? What a strong answer covers Parameters are contravariant and return types covariant in sound reasoning; getting it wrong causes unsafe assignments. They can reason about assignability. Red flag No concept of variance; assigns incompatible function types. ### How do you keep type-checking fast in a large repo? What a strong answer covers Project references, incremental builds, avoiding pathological conditional types, and typing boundaries rather than everything. 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