infer
Declares a type variable to be inferred within a conditional type.
Syntax
T extends SomePattern<infer U> ? U : neverExamples
type ElementOf<T> = T extends (infer U)[] ? U : never;
type N = ElementOf<number[]>; // number
type Return<T> =
T extends (...args: any[]) => infer R ? R : never;
type R = Return<() => string>; // string
Notes
`infer` can only appear in the `extends` clause of a conditional type. It
introduces a fresh type variable captured by pattern matching. Multiple
`infer` positions for the same variable in a covariant position produce a
union; in a contravariant position they produce an intersection.