实现一个累加函数的功能比如 sum(1,2,3)(2).valueOf() ​

function sum(...args) {
  let result = 0;

  result = args.reduce(function (pre, item) {
    return pre + item;
  }, 0);

  let add = function (...args) {
    result = args.reduce(function (pre, item) {
      return pre + item;
    }, result);

    return add;
  };

  add.valueOf = function () {
    console.log(result);
  };

  return add;
}