事件的兼容写法

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const on = (function() {
if (document.addEventListener) {
return function(element, event, handler) {
element && event && handler && element.addEventListener(event, handler, false);
};
} else {
return function(element, event, handler) {
element && event && handler && element.attachEvent('on' + event, handler);
};
}
})();


const off = (function() {
if (document.removeEventListener) {
return function(element, event, handler) {
element && event && element.removeEventListener(event, handler, false);
};
} else {
return function(element, event, handler) {
element && event && element.detachEvent('on' + event, handler);
};
}
})();