SOURCE

      var arr = [1,2,3,4,5];
      var arr2 = [];
      arr[arr.length] = 6;
    //   arr[arr.length] = 7; 暂时最优解
    // push只在移动端性能等于arr[arr.length]
    //   arr2 = arr.concat([6]); 慢20%-50%
      console.log(arr)