js题，前一部分的优化

1、

<script>
    var myArr = [1,2,3,4];
    var item = myArr.indexOf(3);
    document.write(item);
</script>

2、

<script>
  var arr = [1,2,3,4];
  var sum = arr.reduce(function(prev,cur){
      return prev + cur;
  });
  document.write(sum);
</script>

3、

<script>
    var arr = [1,2,3,4,2];
    var item = 2;
    var arr1 = arr.filter(function (item, p2, p3) { return item!=2;  })
    document.write(arr1);
</script>

4、

<script>
    var arr = [1,2,3,4,2];
    var item = 2;
    for (var i=0;i<=4;i++){
        if (arr[i] === item){
            arr.splice(i,1)
        }
    }
    document.write(arr)
</script>