<!DOCTYPE html>
<html>
<body>
<h1>使用 "use strict"：</h1>
<h2>不允许写入只读属性。</h2>
<p>在浏览器中激活调试（F12）以查看错误报告。</p>
<script>
"use strict";
var obj = {};
Object.defineProperty(obj, "x", {value:0, writable:false});
obj.x = 3.14; // 会引发错误
</script>
</body>
</html>