code: function (it, S) {
    if (!_isObject(it)) return it;
    var fn, val;
    if (S && typeof (fn = it.toString) == 'function' && !_isObject(val = fn.call(it))) return val;
    if (typeof (fn = it.valueOf) == 'function' && !_isObject(val = fn.call(it))) return val;
    if (!S && typeof (fn = it.toString) == 'function' && !_isObject(val = fn.call(it))) return val;
    throw TypeError("Can't convert object to primitive value");
  }
docs: instead of the ES6 spec version, we didn't implement @@toPrimitive case and the second argument - flag - preferred type is a string