<script type="text/javascript">
var numA=document.getElementById("num_a").value;
var numB=document.getElementById("num_b").value;
document.getElementById("result").innerHTML=Number(numA)+Number(numB);
<p>使用 output 元素的简易计算器:</p>
<form onsubmit="return false">
<output id="result"></output>
<input type="button" onclick="resCalc()" value="提交"></button>