Awaited type under the hood in TypeScript

1type Awaited<T> = any; // implementation
2
3type Step1 = Awaited<Promise<Promise<string | undefined>>>;
4type Step2 = Awaited<Promise<string | undefined>>;
5type Result = string | undefined;

Seventh challenge is Awaited

It's available since TypeScript 4.5 🔥🔥🔥

Like await in JavaScript, it unwraps Promise and gets the value in the same way.

Sometimes it's useful to get the value even if it's double wrapped with Promise so this is also possible.

Unboxing values from types

Type inference in conditional types is defined in TypeScript as following:

Within the extends clause of a conditional type, it is now possible to have infer declarations that introduce a type variable to be inferred. Such inferred type variables may be referenced in the true branch of the conditional type

1type ReturnType<T> = T extends (...args: any[]) => infer R ? R : any;

As we want to unbox double or maybe thrice wrapped Promise we need Recursive Conditional Types:

1type ElementType<T> = T extends ReadonlyArray<infer U> ? ElementType<U> : T;

It's available since TypeScript 4.1 and allows us to do so with Promise too:

1type Awaited<T> = T extends Promise<infer U> ? Awaited<U> : T;

This is it ⭐️

Don't forget to check the solution on Playground – https://tsplay.dev/mqQxkm 🚀