4.反转数组

<script type="text/javascript">
    var arr = [1,2,3,4,5,6,7,8];
    //定义一半的数组元素，让其与后一半互换位置
    for(var i = 0;i<arr.length/2;i++){
        //arr[0]    arr[arr.length-1-0];
        //arr[1]    arr[arr.length-1-1];
        //arr[2]    arr[arr.length-1-2];
        //arr[3]    arr[arr.length-1-3];
                
        //借助第三方变量交换两个变量的值
        var temp = arr[i];
        arr[i] = arr[arr.length-1-i];
        arr[arr.length-1-i] = temp;
    }
    console.log(arr);
</script>

<script type="text/javascript">
    var arr = [1,2,3,4,5,6,7,8];
    //reverse() 反转元素
    console.log(arr.reverse());
</script>