Cross-browser function for adding an event listener

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2 min. reading

Here is a function that should work in virtually every browser, even really old ones. Of course, it is only useful, if you don’t use jQuery. Because, if you do, a far better way is to just call .on() or .bind() or .delegate() or .live() or whatever you want, and let jQuery do all the dirty work.

Function code:

/**
* Adds event handler; works for all browsers, including old IE versions.
*
* @param event Event name (string) to attach.
* @param thefunction Function executed upon attached event fire (callback).
*/
function addListener(event, thefunction)
{
    if(window.addEventListener)
    {
        //All browsers, except IE before version 9.
        window.addEventListener(event, thefunction, false);
    } 
    else if(window.attachEvent)
    {
        //IE before version 9.
        event = (event == 'DOMContentLoaded') ? 'onreadystatechange' : "on" + event;
        window.attachEvent(event, thefunction);
    }
    else
    {
        /**
        * Code for really old ones -- like NS4 or IE5Mac, which
        * neither support attachEvent nor addEventListener
        * 
        * This code is copyright 2003 by Gavin Kistner, !@phrogz.net
        * It is covered under the license viewable at http://phrogz.net/JS/_ReuseLicense.txt
        * 
        * More info: http://phrogz.net/JS/AttachEvent_js.txt
        */
        if(!window.myEvents) window.myEvents = {};
        if(!window.myEvents[event]) window.myEvents[event] = [];

        var events = window.myEvents[event];
        events[events.length] = thefunction;

        window['on' + event] = function()
        {
            if(!window || !window.myEvents || !window.myEvents[event]) return;

            var events = window.myEvents[event];

            for(var i = 0, len = events.length; i<len; i++) events[i]();
        };
    }
}

Because I don’t have access to those “really old ones”, I cannot test and guarantee, that it will work there. Example used here comes from 2003, so it is itself “really old one”! :]

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