What does infer mean in TypeScript?

TypeScriptSenior~1 min read

Syntax

javascript

T extends SomeType<infer U> ? U : DefaultType

infer U is a declaration of a new type U, which TypeScript will try to infer from the type T, if T matches the pattern SomeType<...>.
If it can be inferred, U is returned.
If it cannot, the branch after : is used.

Example 1: Extracting the type from `Promise`

javascript

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

type A = UnwrapPromise<Promise<string>>; // string
type B = UnwrapPromise<number>;          // number

Here infer U says: "if T is Promise<X>, infer X".

Example 2: Extracting the array element type

javascript

type ElementType<T> = T extends (infer U)[] ? U : T;

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

infer U lets you "unwrap" the type of array elements.

Example 3: Extracting a function's argument type

javascript

type ArgType<T> = T extends (arg: infer U) => any ? U : never;

type A = ArgType<(x: number) => void>; // number
type B = ArgType<(x: string) => void>; // string

If T is a function, infer U pulls out the type of its argument.

Example 4: Extracting a function's return type

javascript

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

type A = ReturnTypeOf<() => string>; // string
type B = ReturnTypeOf<(x: number) => boolean>; // boolean

This is exactly how the built-in ReturnType<T> utility type is implemented in TypeScript.

Example 5: Multiple `infer` extractions

javascript

type FirstArgAndReturn<T> =
  T extends (arg: infer A) => infer R ? [A, R] : never;

type Example = (x: string) => number;

type Result = FirstArgAndReturn<Example>; // [string, number]

Example 6: Extracting types from tuples

javascript

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

type A = First<[1, 2, 3]>; // 1
type B = Last<[1, 2, 3]>;  // 3

Example 7: Unwrapping nested types (nested generics)

javascript

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

type A = DeepUnwrap<Promise<Promise<string>>>; // string

How this works logically

infer operates inside a conditional type:

It checks whether T matches a certain structure (extends Pattern).
If it does, TypeScript "substitutes" concrete types in place of infer U.
These inferred types can be used on the right-hand side of the expression (? ... : ...).

Important limitations

infer only works inside extends (it cannot be used on its own).
You can declare several infer variables.
TypeScript can infer a union of types if the condition matches several forms.

SUMMARY

What it does	Example	Result
Extracts the type from `Promise`	`T extends Promise<infer U> ? U : T`	"unwraps" the promise
Extracts an array element	`T extends (infer U)[] ? U : T`	gets the element type
Extracts a function argument	`T extends (arg: infer A) => any ? A : never`	gets the argument type
Extracts the return value	`T extends (...args: any[]) => infer R ? R : never`	gets the result type
Extracts from a tuple	`T extends [infer F, ...any[]] ? F : never`	the first element

Short Answer

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