Implement keyBy(arr, keyFn)
Like groupBy, but for the common case where each key is expected to be unique, write keyBy(arr, keyFn) that returns an object mapping each key to its single matching item (not an array). If two items share a key, the later one wins.
keyBy([{ id: 1, name: "Ada" }, { id: 2, name: "Grace" }], (u) => u.id);
// { 1: { id: 1, name: "Ada" }, 2: { id: 2, name: "Grace" } }