点击弹出当前index,这段js用闭包该怎么写？

<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>无标题文档</title>
<style>
li { height:30px; border-bottom:1px solid #333; }
</style>
<script>
window.onload = function (){
    var oUl = document.getElementById('list');
    var aLi = oUl.getElementsByTagName('li');
    var arr = [ '今天', '明天', '后天' ];
    var len = arr.length;
    
    for( var i=0; i<len; i++ ){
        aLi[i].innerHTML = arr[i];
        aLi[i].index=i;
        aLi[i].onclick = function (){
            alert(this.index)
        };
    }
};
</script>
</head>

<body>
<ul id="list">
    <li></li>
    <li></li>
    <li></li>
</ul>
</body>
</html>

var li=document.querySelectorAll('li'),
    ul=document.querySelector('ul');
[].forEach.call(li,function(v,i){
    v.index=i
})

ul.addEventListener('click',function(e){
    var target=e.target
    if(target.nodeName==='LI'){
        alert(target.index)
    }
},false)

aLi[i].onclick = (function(index) {
    return function() {
        alert(index);
    };
})(i);

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