惰性函数

场景：不用每次都对同一个东西进行判断

function addEvent (type, el, fn) {
    if (window.addEventListener) { //这样不是很次进来都会对window.addEventListener进行判断
        addEvent = function (type, el, fn) {
            el.addEventListener(type, fn, false);
        }
    }
    else if(window.attachEvent){
        addEvent = function (type, el, fn) {
            el.attachEvent('on' + type, fn);
        }
    }
}

addEvent();

 

来源：oschina

链接：https://my.oschina.net/u/2285087/blog/3157644