What do ReturnType and Parameters do? | Interview Question

Q: What do ReturnType and Parameters do?

**Target Audience:** Developers who keep writing the same type twice — once on a function, once next to it — and want to derive it instead. **Difficulty:** Medium **TypeScript version:** Every type claim below was checked by TypeScript 5.4.5 with `--strict`; release versions are cited, not run. > **How to read this doc:** Every idea is explained in plain language first, then tagged with `📌 Interview term:` so you also learn the exact words an interviewer expects. Type claims are checked by `tsc` (an `Equal ` line or a `// @ts-expect-error` line), and the **Verified** section quotes the real output in full. ## 1. Why This Even Matters — A Story First You buy a coffee machine with no manual. You can still learn everything that matters by looking at it: the slots on the top tell you what goes **in** (water, beans, a pod), the spout tells you what comes **out** (coffee). You do not need the factory to send you a separate spec sheet — the machine itself is the spec. A function is t

The problem

Question presented to candidate: "What do ReturnType and Parameters do? When have you used them, and what are their limits — say with overloads or generic functions?"

What a strong answer should cover:

ReturnType<F> gives the return type of a function type; Parameters<F> gives its parameter types as a (labelled) tuple; both need a function type, so you pass typeof fn
They are conditional types with infer (F extends (...args: any) => infer R ? R : any)
Uses: name a type nobody exported, type wrappers and decorators that keep a signature ((...args: Parameters<F>) => ReturnType<F>), forward arguments; ConstructorParameters and InstanceType for classes
Async functions: ReturnType is the Promise; combine with Awaited for the resolved value
Limits: overloads use only the last signature, generic functions get their constraint (unknown) unless you pin the type with an instantiation expression, and non-functions are rejected by the constraint

Code / implementation expected: Optional — a withLog wrapper using both, or MyReturnType with infer.

returntype
parameters

Try it yourself

Sketch your approach before you read the solution. That is what the interview tests.

Stuck? Get a hint.

A short nudge toward the approach. It will not give the answer away.

Solution

Target Audience: Developers who keep writing the same type twice — once on a function, once next to it — and want to derive it instead. Difficulty: Medium TypeScript version: Every type claim below was checked by TypeScript 5.4.5 with --strict; release versions are cited, not run.

How to read this doc: Every idea is explained in plain language first, then tagged with 📌 Interview term: so you also learn the exact words an interviewer expects. Type claims are checked by tsc (an Equal line or a // @ts-expect-error line), and the Verified section quotes the real output in full.

1. Why This Even Matters — A Story First

You buy a coffee machine with no manual. You can still learn everything that matters by looking at it: the slots on the top tell you what goes in (water, beans, a pod), the spout tells you what comes out (coffee). You do not need t

The solution is hidden so you can try first

9 min read · 10 sections

Run the code

Edit the code and run it here. Each console line with an expected value becomes a test.

TypeScriptDeriving types from functions and typing a wrapper

Practice

TypeScriptOverloads, generic functions, methods and other edge cases

Practice

Discussion

Other approaches, follow-ups and how it went in real interviews.

Sign in to join the discussion.

No comments yet. Share how you would answer it.

The solution is hidden so you can try first

Keep your momentum going.