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 } 

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Original answer

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  

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