只需要第一次判断条件就能确定，后面再多次调用时就不需要重新判断条件了。
function addEvent(type
, element
, fun
) {
if (window
.addEventListener
) {
addEvent = function (type
, element
, fun
) {
element
.addEventListener(type
, fun
, false);
};
} else if (window
.attachEvent
) {
addEvent = function (type
, element
, fun
) {
element
.attachEvent('on' + type
, fun
);
};
} else {
addEvent = function (type
, element
, fun
) {
element
['on' + type
] = fun
;
};
}
addEvent(type
, element
, fun
)
}
