<html>
<body>
<p>x = 10 ， y = 5, 计算 x += y, x 为:</p>
<button onclick="myFunction()">点我</button>
<p id="demo"></p>
<script>
function myFunction() {
var x = 10;
var y = 5;
x += y;
document.getElementById("demo").innerHTML = x;
}
</script>
</body>
</html>