Cross-browsers events listener implementation

gistfile1.js

({
    addListener: null,
    removeListener: null,
    
    init: function () {
        if (typeof window.addEventListener === 'function') {
            this.addListener = function (el, type, fn) {
                el.addEventListener(type, fn, false);
            };
            this.removeListener = function (el, type, fn) {
                el.removeEventListener(type, fn, false);
            };
        } else if (typeof document.attachEvent === 'function') { // IE
            this.addListener = function (el, type, fn) {
                el.attachEvent('on' + type, fn);
            };
            this.removeListener = function (el, type, fn) {
                el.detachEvent('on' + type, fn);
            };
        } else { // older browsers
            this.addListener = function (el, type, fn) {
                el['on' + type] = fn;
            };
            this.removeListener = function (el, type, fn) {
                el['on' + type] = null;
            };
        };
        
        return this;
    }
}).init()