<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title></title>
</head>
<body>
<input type="text" value="" id="num1"/>
<span>+</span>
<input type="text" value="" id="num2"/>
<span>=</span>
<input type="text" value="" id="num3"/>
</body>
</html>
<script>
var oNum1 = document.getElementById( "num1" );
var oNum2 = document.getElementById( "num2" );
var oNum3 = document.getElementById( "num3" );
//判断num1和num2是否为空
function num(){
if( oNum1.value == "" || oNum2.value == ""){
oNum3.value = "输入的数值不能为空"
}else{
oNum3.value = Math.sqrt(Math.pow((oNum1.value),2)+Math.pow(Number(oNum2.value),2));
}
}
oNum1.onfocus = function(){
num();
oNum2.onfocus = function(){
num();
}
}
oNum2.onkeyup = function(){
num();
oNum1.onkeyup = function(){
num();
}
}
</script>