计数器实现方式

<!DOCTYPE html>
<html>
<head lang="en">
    <meta charset="UTF-8">
    <title>计数器</title>
</head>
<body>
    <button id="sub">-</button>
    <span id="num"></span>
    <button id="add">+</button>
<script type="text/javascript" src="">

(function(){
    var subBtn=document.getElementById('sub');
    var span=document.getElementById('num');
    var addBtn=document.getElementById('add');

    if(localStorage.num===undefined){
        localStorage.num=0;
    }

    span.innerHTML=localStorage.num;

    subBtn.addEventListener('click',function(){
        localStorage.num--;
        span.innerHTML=localStorage.num;
    },false);

    addBtn.addEventListener('click',function(){
       localStorage.num++;
        span.innerHTML=localStorage.num;
    },false);

})();


</script>
</body>
</html>