前端笔试题：数组求和的方式

 <script>
    Array.prototype.sum = function(){
      var sum = 0;
      //构造函数里面的this指向实例对象
      for(var i = 0;i < this.length;i++){
        sum += this[i];
      }
      return sum;
    }
    //这种写法就会覆盖掉Array原有的方法
    // Array.prototype = {
    //   sum:function(){
    //     for(var i = 0;i < this.length;i++){
    //     sum += this[i];
    //   }
    //   return sum;
    //   }
    // }
    var arr = [1,2,3];
    console.log(arr.sum());
    var arr1 = new Array(11,22,33);
    console.log(arr1.sum());
  </script>

  //forEach 迭代
    var arr = [1,2,3];
    var sum = 0;
    arr.forEach(function(value,index,array){
      console.log(value,index,array);
      sum += value;
    })
    console.log(sum);