inference

`infer R`

A keyword usable only inside the extends clause of a conditional type that introduces a fresh type variable inferred from the matched shape. It is the primitive that powers ReturnType, Parameters, InstanceType, Awaited, and countless custom utilities.

Definition

T extends SomeShape<infer R> ? R : Fallback

Usage patterns

Pattern	Purpose
infer R	Introduce a fresh inferred type variable
extends (...a: any) => infer R	Infer a function's return type
extends Array<infer E>	Infer an element type from an array
extends `${infer H}-${infer T}`	Infer parts from a template literal type

Examples

type ElementOf<T> = T extends (infer E)[] ? E : never;
type E = ElementOf<number[]>;

Result: number — infer used on tuple/array shapes

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

Grabs the head of a tuple with rest inference

type Split<S> = S extends `${infer H}.${infer T}` ? [H, ...Split<T>] : [S];

Recursive string-splitting via template literal inference

type R = (() => number) extends () => infer X ? X : never;

R is `number` — the classic ReturnType pattern. Without the parens the arrow parses as a function type whose body is the conditional, giving `() => never` instead.

Gotcha

Only valid inside the `extends` clause of a conditional type — using `infer` elsewhere is a syntax error. Multiple `infer` sites for the same name in co-variant positions yield a union; in contra-variant positions they yield an intersection.

Related utility types

ReturnType<T>

Infers the return type of a function type T using `infer R`. Widely used to derive types from existing functions without redeclaring them.

Parameters<T>

Extracts the parameters of a function type T as a tuple. Enables reusing a function's argument list for wrappers, currying, or higher-order utilities.

Awaited<T>

Models the type produced by awaiting T, recursively unwrapping nested Promises (and other thenables). Introduced in TS 4.5 and used internally by the `await` expression's type.

InstanceType<T>

Given a constructor type T (typically obtained via `typeof ClassName`), returns the type of an instance produced by `new T()`. Useful for factory functions and generic wrappers over classes.

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