封闭函数和闭包

闭包

function aa(b){
    var a = 12;
    function bb(){
          alert(a);
          alert(b);
    }
     return bb;
    }
    var cc = aa(24);

闭包就是函数里面由函数,也可以交函数的嵌套

闭包的用途

1.存索引

li{
       height:30px;
       background-color :red;
        margin-bottom:10px;  
}
</style>
<script type="text/javascript">
        window.onload = functin(){
            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);