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 p

Answer 1

You can use conditional types in Tyepscript 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