What does the infer keyword do?

Q: What does the infer keyword do?

**Target Audience:** Developers who know generics and have seen a conditional type (`T extends U ? X : Y`), and want to extract parts of types. **Difficulty:** Hard **TypeScript version:** Every type claim below was checked by TypeScript 5.4.5 with `--strict`; release facts (2.8, 4.7, 4.8) 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 A postal worker sorts envelopes with a stencil. The stencil has a window cut out where the postcode goes: "street, town, **[____]**". She lays it over each envelope. If the envelope fits the stencil, whatever shows through the window is the postcode, and she can write it down. If it does not fit (a parcel, a postcard

The problem

Question presented to candidate: "What does the infer keyword do in TypeScript? Show me how you would use it to pull a type out of another type, like the return type of a function."

What a strong answer should cover:

infer X declares a new type variable inside the extends clause of a conditional type; TypeScript fills it in by matching the checked type against the pattern, and X is usable in the true branch
If the pattern does not match, the false branch is taken; infer is only legal in the extends clause (error TS1338 elsewhere)
It can sit in any position: return type, parameters, array element, tuple head or rest, a generic argument like Promise<infer V>, or a hole in a template literal type
Built-ins such as ReturnType, Parameters and InstanceType are written with it (added with conditional types in TypeScript 2.8)
Finer rules: one name in several covariant spots gives a union, in parameter spots an intersection; infer X extends C (TypeScript 4.7) constrains the match; overloaded functions infer from the last signature

Code / implementation expected: Yes — write a MyReturnType<F> or ElementOf<T> with infer, and explain the false branch.

infer

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 know generics and have seen a conditional type (T extends U ? X : Y), and want to extract parts of types. Difficulty: Hard TypeScript version: Every type claim below was checked by TypeScript 5.4.5 with --strict; release facts (2.8, 4.7, 4.8) 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

A postal worker sorts envelopes with a stencil. The stencil has a window cut out where the postcode goes: "street, town, [____]". She lays it over each envelope. If the envelope fits the stencil, whatever shows through t

The solution is hidden so you can try first

11 min read · 10 sections

Run the code

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

TypeScriptinfer in every position, the false branch and where infer is not allowed