TypeScript keyof returning specific type

Question

If I have the following type

interface Foo {
    bar: string;
    baz: number;
    qux: string;
}

can I use typeof to type a parameter such that it only takes keys of Foo that return string ('bar' or 'qux')?

Answer 1

Typescript 4.1 and above

With the addition of the as clause in mapped types we can now simplify the original type to:

type KeyOfType<T, V> = keyof {
    [P in keyof T as T[P] extends V? P: never]: any
}

Playground Link

Original answer

You can use conditional types in Typescript 2.8 :

type KeysOfType<T, TProp> = { [P in keyof T]: T[P] extends TProp? P : never }[keyof T];

let onlyStrings: KeysOfType<Foo, string>;
onlyStrings = 'baz' // error
onlyStrings = 'bar' // ok

let onlyNumbers: KeysOfType<Foo, number>;
onlyNumbers = 'baz' // ok
onlyNumbers = 'bar' // error 

Playground Link