例子：闯关得分

<script type="text/javascript">
var guan =prompt("输入关数","关数");
var defen =0;
if(guan!=null&&guan!="")
{
    if(isNaN(guan)==false&&guan>0&&guan<=50&&guan%1==0)
    {
        if(guan<=20)
        {
            for(i=0;i<=guan;i++)
            {
                defen=defen+i;
            }
            alert(defen)
        }
        else if(guan<=30)
        {
            for(j=0;j<guan-20;j++)
            {
                defen=(guan-20)*10+210;
            }
            alert(defen)
        }
        else if(guan<=40)
        {
            for(k=0;k<guan-30;k++)
            {
                defen=(guan-30)*20+310;
            }
            alert(defen)
        }
        else if(guan<=49)
        {
            for(l=0;l<guan-40;l++)
            {
                defen=(guan-40)*30+510;
            }
            alert(defen)
        }
        else
        {
            alert(880);
        }
    }
    else
    {
        alert("你输入的不是一个正确关数")
    }
}
else
{
    alert("不输入关数干啥")
}

</script>