The infer Keyword

Syntax

T extends SomePattern<infer U> ? U : never

Examples

type MyReturnType<T> = T extends (...args: any[]) => infer R ? R : never;

function getUser() {
  return { id: 1, name: "Alice" };
}

type User = MyReturnType<typeof getUser>;
// User is inferred as: { id: number; name: string }

type ElementType<T> = T extends (infer Item)[] ? Item : never;

type A = ElementType<string[]>; // string
type B = ElementType<number[]>; // number

type UnwrapPromise<T> = T extends Promise<infer Value> ? Value : T;
type C = UnwrapPromise<Promise<string>>; // string

Best practices

• Use infer specifically when you need to extract and reuse a piece of a matched type, rather than just checking whether it matches
• Study infer through TypeScript's built-in utility types (ReturnType, Parameters, Awaited) - they are the clearest real-world examples of this pattern
• Remember infer can only appear within the extends clause of a conditional type - it has no meaning outside that specific context
• Keep infer-based types well-named and documented, since the syntax itself is dense and can be hard to parse at a glance for readers unfamiliar with it

At a glance

Purpose

Static types for JavaScript

File extension

.ts · .tsx

Runs in

Compiled to JavaScript

Usually used with

JavaScript and its ecosystem

Specifications & further reading

• TypeScript Handbook

Related TypeScript documentation