在==（相等）判断中，null和undefined相等（它们也与自身相等），除此之外不与其他值相等。
示例代码：
<!DOCTYPE html>
<html lang="zh">
<head>
<meta charset="UTF-8" />
<title>null和undefined相等比较</title>
</head>
<body>
<script type="text/javascript">
var a = null;
var b;
console.log(a == b); //true
console.log(a == null); //true
console.log(b == null); //true
console.log(a == false); //false
console.log(b == false); //false
console.log(a == ‘‘); //false
console.log(b == ‘‘); //false
console.log(a == 0); //false
console.log(b == 0); //false
</script>
</body>
</html>
