Utility types that converts each member of an object type

Question

How can I create a type Promises<T> where T is an object, such that the new type has all the same members of T, and for each member of type T[key] we have instead the type Promise<T[key]>?

Answer 1

You can do this with a mapped type:

type Promises<T extends object> = { [K in keyof T]: Promise<T[K]> }

Let's test it out:

interface Foo {
  a: string;
  b: number;
  c: boolean;
}

type PromisesFoo = Promises<Foo>;
/* type PromisesFoo = {
    a: Promise<string>;
    b: Promise<number>;
    c: Promise<boolean>;
} */

Looks good.

Playground link to code