惰性函数

<div class="box">点击我</div>

<script>
    var oBox = document.getElementsByClassName("box")[0];

    var fun = addEvent(oBox,"click",function(){
        console.log(this);  //oBox元素
    });
    console.log(fun); //addEventListener

    function addEvent(el,type,fn,capture){
        if(el.addEventListener){
            addEvent = function(el,type,fn,capture){
                el.addEventListener(type,fn,capture);
                return 'addEventListener';
            }
        }else if(el.attachEvent){
            addEvent = function(el,type,fn){
                el.attachEvent('on'+type,fn);
                return 'attachEvent';
            }
        }else{
            addEvent = function(el,type,fn){
                el['on'+type] = fn;
                return 'on'
            }
        }
        return addEvent(el,type,fn,capture);
    }
</script>