Object.keys fails on generic types

[millsp]

const clone = <B extends A>(b: B): B => {
    // @ts-ignore
    const clone = (new b.constructor) as B

    for (const key of Object.keys(b)) {
        b[key] // should not error
    }

    return clone
}

The error comes from the fact that .keys() returned string[]. When working with generic types, TypeScript is unsure that a string key will exist within the generic type B (not known). TypeScript can only agree if key is of type keyof B, that explains the change.

[j-oliveras]

This was rejected multiple times: #38328, #34639, #34500, #30314 and others (search Object.keys in closed issues and closed PR)

See: https://stackoverflow.com/questions/55012174/why-doesnt-object-keys-return-a-keyof-type-in-typescript

Simple example that is a bad idea:

const a = {
        c: 1,
        d: 2,
};

const b: { c: number } = a;

Object.keys(b); // With your types that will be ("c")[], but is ("c" | "d")[]

[millsp]

Thanks @j-oliveras