js.js

Is(constructor, value) {
  if (constructor == null || value == null) {
    return false;
  }

  let C = value.[[Constructor]];
  if (C === undefined) {
    return false;
  }

  do {
    if (C === constructor) {
      return true;
    }
    if (C.[[ConstructorKind]] !== 'derived') {
      return false;
    }
    C = C.[[ConstructorParent]];
  } while (C);

  return false;
}