Recursive Conditional Types

confitional typesで再帰ができるようになった

code:ts

type ElementType<T> =

T extends ReadonlyArray<infer U> ? ElementType<U> : T;

function deepFlatten<T extends readonly unknown[]>(x: T): ElementType<T>[] {

throw "not implemented";

}

// All of these return the type 'number[]':

Similarly, in TypeScript 4.1 we can write an Awaited type to deeply unwrap Promises.

code:ts

type Awaited<T> = T extends PromiseLike<infer U> ? Awaited<U> : T;

declare function customThen<T, U>(

p: Promise<T>,

onFulfilled: (value: Awaited<T>) => U

): Promise<Awaited<U>>;