简易计算器
超短源代码：
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>简易计算器</title>
<script>
function lst() { return Number(document.getElementById('lst').value); }
function rst() { return Number(document.getElementById('rst').value); }
function ans(v) { document.getElementById('ans').value = v; }
</script>
</head>
<body>
<h1>简易计算器</h1>
<label for="lst">第一个数</label> <input id="lst" type="number"><br>
<label for="rst">第二个数</label> <input id="rst" type="number"><br>
<input type="button" value="+" onclick="ans(lst()+rst())"> 
<input type="button" value="-" onclick="ans(lst()-rst())"> 
<input type="button" value="*" onclick="ans(lst()*rst())"> 
<input type="button" value="/" onclick="ans(rst()?lst()/rst():'除数不得为零')"><br>
<label for="ans">计算结果</label> <input id="ans" type="text" readonly><br>
</body>
</html>