闭包

闭包就是函数里面有函数，也可以叫函数的嵌套

  function aa(b) {
    var a = 12;
    function bb() {
        alert(a);
        alert(b);
    }
    return bb;
}
var cc = aa(24);
 //封闭函数写闭包 
 /*var cc =(function (b) {
     var a = 12;
    function bb() {
        alert(a);
        alert(b);
    }
    return bb;
 })(24);*/
cc()

闭包的用途

1.存索引

li{
    height: 30px;
    background-color: gold;
    margin-bottom: 10px;
}
  </style>
<script type="text/javascript">
 window.onload = function () {
    var ali = document.getElementsByTagName('li');
    
    for(var i=0; i<ali.length; i++){
        (function (i) {
            ali[i].onclick = function () {
                alert(i)
            }
        })(i)
    }
}

2.私有计数器

 var count = (function () {
    var a = 0;
    function bb() {
        a++;
        return a;
    }
    return bb;
})();
alert(count());
alert(count());
var c = count();
alert(c);