//for循环正序 and 倒序
for(var i=0;i<arr.length;i++)
{
   alert("Hello,"+arr[i]+"!");
};
for(var i=arr.length-1;i>=0;i--)
{
   alert("Hello,"+arr[i]+"!");
}
//while循环正序 and 倒序
var n = 0;
while(n<arr.length)
{
   alert("Hello,"+arr[n]+"!");
   n++;
}
var j = arr.length-1;
while(j>=0)
{
   alert("Hello,"+arr[j]+"!");
   j--;
} 在此插入代码