惰性函數(shù)：函數(shù)執(zhí)行的分支只會(huì)在函數(shù)第一次調(diào)用的時(shí)候執(zhí)行，在第一次調(diào)用的過(guò)程中，改函數(shù)被覆蓋為另一個(gè)按照合適的方式執(zhí)行的函數(shù)，這樣任何對(duì)原函數(shù)的調(diào)用就不用再經(jīng)過(guò)執(zhí)行的分支了。

addEventListener 事件函數(shù)封裝

var addEvent = function(el, type, fn, capture) {
    if(el.addEventListener) {
        addEvent = function(el, type, fn, capture) {
            el.addEventListener(type, fn, capture);
        }
    } else if(el.attachEvent) {
        addEvent = function(el, type, fn) {
            el.attachEvent('on' + type, function() {
                fn.call(el);
            })
        }
    } else {
        addEvent = function(el, type, fn) {
           el['on' + type] = fn;
        }
    }
    addEvent(el, type, fn, capture);
}