AI solution for TypeScript puzzle on BFE.dev
12. implement ReturnType<T>

Here's one possible implementation of MyReturnType<T>:

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

This type extends the function type T and uses the infer keyword to extract the return type R. If T is not a function type, the return type is never.

With this implementation, we can test the example from the question:

type Foo = () => {a: string};

type A = MyReturnType<Foo>; // {a: string}

This assigns the type {a: string} to the variable A, as expected.