var sumNum=0//记录和的变量
for(var i=1i<=100i++){
sumNum+=i//累加上次的值
}
alert('1+2+3+....+99+100='+sumNum)//输出和
}
<script>function dd(){for(var i=0i<2i++){var op+="sdfsd"}
alert(op)
}
dd()</script>
<input type="text" id="a" onchange="s()" />+<input type="text" id="b" onchange="s()" />+
<input type="text" id="c" onchange="s()" />=
<input type="text" id="d" readonly="readonly" />
<script type="text/javascript">
function s(){
var $ = document.getElementById
var a = $("a")
var b = $("b")
var c = $("c")
var d = $("d")
if(isNaN(a.value)){alert("请输入数字！")a.focus()return}
if(isNaN(b.value)){alert("请输入数字！")b.focus()return}
if(isNaN(c.value)){alert("请输入数字！")c.focus()return}
d.value = a.value * 1 + b.value * 1 + c.value * 1
return
}
</script>
