自执行函数 块级作用域
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8" />
<meta name="viewport" content="width=device-width, initial-scale=1.0" />
<meta http-equiv="X-UA-Compatible" content="ie=edge" />
<title>Document</title>
</head>
<body>
<script type="text/javascript">
var a = 1;
(function(){
var a = a;
console.log(a);//undefined
})()
var c = 1;
(function(){
var b = c;
console.log(b); //0
})()
var d = 1;
(function(){
console.log(d); //0
})(d)
</script>
</body>
</html>
