code: function copyOnto(a1, a2) {
    if (Array.isArray(a1) && Array.isArray(a2)) return copyOntoArray (a1, a2);
    if (isObject     (a1) && isObject     (a2)) return copyOntoObject(a1, a2);
    return (a1 = a2);
  }
docs: Copy a second object or array destructively onto a first one.