"use strict";
//严格模式
<!doctype html>
<html>
<head>
<meta charset="utf-8">
<title>无标题文档</title>
</head>
<body>
<script>
"use strict";
(function() {
var a = b = 5;
})();
//这个会报错哦;
console.log(b);
</script>
</body>
</html>
<!doctype html>
<html>
<head>
<meta charset="utf-8">
<title>无标题文档</title>
</head>
<body>
<script>
(function() {
var a = b = 5;
})();
//这个居然跑出来一个5,尼玛!!
console.log(b);
</script>
</body>
</html>