text: Array.prototype._map = function (fn) {
    if (typeof fn !== 'function') {
      throw Error('参数必须是一个函数');
    }
  
    let res = [];
    for (let i = 0; (len = this.length), i < len; i++) {
      res.push(fn(this[i]));
    }
    return res;
  };
  