Filter keyof properties by types

[asfernandes]

So there is:

type Deferred<T> = {
    [P in keyof T]: Promise<T[P]>;
};

And we have:

interface Obj
{
    id: number;

    employee: {
        id: number;
        name: string;
    }

    // an array here too
}

With Deferred<Obj> the employee object and arrays will also be Promise but that would not always be what we want. It also will not nest on children objects.

The ability to filter keys by type would be very good as:

type Container<T> = {
    [P in keyof T where T[P] extends number | string]: Promise<T[P]>;
    [P in keyof T where T[P] extends {}]: Container<T[P]>;
}

If use more than one in would be out of question, something using intersection may work too.

[ahejlsberg]

Yes, this would be a nice capability to have. I think it is already covered by #12424.

[asfernandes]

I understand the similarity and depending on the design of #12424 it will cover my needs. On the other hand, this issue may do additional things, i.e., really "filter" out (hide) what comes from in/keyof depending on a condition.