Immer-like updates with explicit keys

update.ts

// Modify an object, passing an explicit key array. The callback only has access to the keys in the array.
function modify<T, K extends keyof T>(instance: T, keys: K[], update: (v: Pick<T, K>) => void) {
  const draft = structuredClone(instance);
  update(draft);
  // for example's sake, just log the changed keys
  for (let k of keys) {
    console.log("updated", k, "from", instance[k], "to", draft[k]);
  }
}

const value = Object.freeze({a: 1, b: "4", c: false});
modify(value, ["a", "b"], draft => {
  draft.a = 2;
  // NOT ALLOWED
  //draft.c = true;
});