懒惰模式：减少每次代码执行时的重复性的分支判断，通过对对象重定义来屏蔽原对象中的分支判断。
<script>
// 需要重复进入分支，浪费资源
var A = {}
A.on = function (dom, type, fn) {
if (dom.addEventListener) {
dom.addEventListener(type, fn, false)
} else if (dom.attachEvent) {
dom.attachEvent('on' + type, fn)
} else {
dom['on' + type] = fn
}
}
// 通过闭包，实现加载即执行
A.on = function (dom, type, fn) {
if (document.addEventListener) {
return function (dom, type, fn) {
dom.addEventListener(type, fn, false)
}
} else if (document.attachEvent) {
return function (dom, type, fn) {
dom.attachEvent('on' + type, fn)
}
} else {
return function (dom, type, fn) {
dom['on' + type] = fn
}
}
}
// 懒惰执行
A.on = function (dom, type, fn) {
if (dom.addEventListener) {
A.on = function (dom, type, fn) {
dom.addEventListener(type, fn, false)
}
} else if (dom.attachEvent) {
A.on = function(dom, type, fn) {
dom.attachEvent('on' + type, fn)
}
} else {
A.on = function (dom, type, fn) {
dom['on' + type] = fn
}
}
}
</script>
