TypeScript Promise functor

promiseFunctor.ts

function PromiseFunctor<In, Out>(f: (input: In) => Out, value: Promise<In>): Promise<Out> {
    return value.then<Out>(
        a => f(a)
    );
}

// ==== TEST ====

const x = 1;
const p = Promise.resolve(x);

PromiseFunctor(y => String(y), p)
    .then(y => {
        console.log(y === x.toString())
    });