README.md

infer in conditional types

The infer keyword is used within conditional types to infer (extract) type information. It allows you to capture and use types that are matched within a conditional type.

type InferredType<T> = T extends infer U ? U : never;

Here, U is a type variable that is inferred from T.

Usages

infer is particularly useful when working with complex types, such as function types, promise types, or nested object types.

A lot of useful built-in helper types in TypeScript are actually built using infer under the hood.

Inferring return types

You can use infer to extract the return type of a function:

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

type Func = (x: number) => string;
type FuncReturnType = MyReturnType<Func>; 
//   ^ FuncReturnType: string

Inferring Promise return types

You can extract the type wrapped by a Promise:

type UnwrapPromise<T> = T extends Promise<infer U> ? U : T;

type PromiseType = UnwrapPromise<Promise<number>>; // number

Inferring array element Types

You can extract the element type of an array:

type ArrayElement<T> = T extends (infer U)[] ? U : never;

type NumberArray = ArrayElement<number[]>; // number

Multiple inferences

You can use multiple infer keywords in a single conditional type:

type FirstAndLast<T extends any[]> = T extends [infer F, ...any[], infer L] ? [F, L] : [never, never];

type FL = FirstAndLast<[1, 2, 3, 4]>; // [1, 4]

The infer keyword, combined with conditional types, provides a powerful way to perform type-level programming in TypeScript, enabling complex type transformations and pattern matching on types.