惰性加载函数

常见写法

let addEvent = (ele, type, handler) => {
    if (window.addEventListener) {
        return ele.addEventListener(type, handler, false);
    }
    if (window.attachEvent) {
        return ele.attachEvent(`on${type}`, handler);
    }
};

嗅探

let addEvent = (() => {
    if (window.addEventListener) {
        return function (ele, type, handler) {
            ele.addEventListener(type, handler, false);
        };
    }
    if (window.attachEvent) {
        return function (ele, type, handler) {
            ele.attachEvent(`on${type}`, handler);
        };
    }
})();

惰性载入

let addEvent = (ele, type, handler) => {
    if (window.addEventListener) {
        addEvent = function (ele, type, handler) {
            ele.addEventListener(type, handler, false);
        };
    }
    if (window.attachEvent) {
        addEvent = function (ele, type, handler) {
            ele.attachEvent(`on${type}`, handler);
        };
    }
    addEvent(ele, type, handler);
};