apply、call、bind ​

Function.prototype._call = function (target, ...args) {
  target = target || window;
  const key = Symbol();
  target[key] = this;
  const res = target[key](...args);
  delete target[key];
  return res;
};

Function.prototype._apply = function (target, args) {
  target = target || window;
  const key = Symbol();
  target[key] = this;
  const res = target[key](...args);
  delete target[key];
  return res;
};

Function.prototype._bind = function (target, ...args) {
  target = target || window;
  const key = Symbol();
  target[key] = this;
  return function (...innerArgs) {
    const res = target[key](...args, ...innerArgs);
    // delete target[key]; // 不能删除，否则第二次调用的时候，就会出现问题。
    return res;
  };
};