Type alias PickByType<T, P>

PickByType<T, P>: {
    [K in Keys<T> as T[K] extends P
        ? K
        : never]: T[K]
}

From T, pick a set of properties whose type are assignable to P.

type T = {
 foo: string,
 bar: bigint | boolean,
 baz: number,
}
PickByType<T,true>; // Result: {}
PickByType<T,number>; // Result: { baz: number }