Assigning dynamically declared event handlers to an object

Discussion in 'Javascript' started by Ron Goral, Nov 17, 2006.

  1. Ron Goral

    Ron Goral Guest


    I am new to creating objects in javascript, so please no flames about my
    coding style. =) I am trying to create an object that will represent a "div"
    element as a menu. I have written several methods that are working fine. The
    problem is if I want to dynamically assign an event handler to the object,
    the event handler is not called. What am I doing wrong? Below is a sample
    HTML doc with everything in place.



    <title>Menu Manager Test</title>
    function testMenu (id)
    this.addAttributes = addProperty;
    this.addEvents = addProperty; = (id || 'mainMenu');
    this.element = document.getElementById(;
    if (this.element == null)
    this.element = document.createElement("<div id='" + +
    "'>Some Text</div>");
    } = 'none';

    function addProperty(propertyName,keyValue)
    // this is a standalone property like an event or innerHTML
    if (propertyName.indexOf("=") >= 0)
    for (var i=0;i<addProperty.arguments.length;i++)
    var arg = addProperty.arguments.split("=");
    this.element[arg[0]] = arg[1];
    // this adds style elements
    // args should be in the form of propertyname=value.
    for (var i=1;i<addProperty.arguments.length;i++)
    var arg = addProperty.arguments.split("=");
    this.element[addProperty.arguments[0]][arg[0]] = arg[1];

    // Sample implementation in the HTML page

    // Test function to check for use.
    function testFunc()
    {alert("here in testFunc.");}

    var menuObj;
    function setMenu(id)
    // Create a new menu
    menuObj = new testMenu(id);
    // Adding a style works
    menuObj.addAttributes("style","border=1px solid black");
    menuObj.addAttributes("innerHTML=A ball of string");
    // Dynamically assigning an event handler does not work.

    <body onLoad="setMenu('mainMenu');">

    Peace in Christ -
    Ron Goral
    Ron Goral, Nov 17, 2006
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  2. Ron Goral

    Daz Guest

    Hi Ron.

    I am a complete novice, but I will give you my 2 cents anyway.

    First of all, I would recommend you save yourself a little bit of
    bandwidth, and consider using CSS as opposed to:
    (This is not a flame, just a suggestion that might make life a little

    As for the mouse events...

    I can think of two mthods (which I cannot vouch for), that you may wish
    to consider trying.
    menuObj.addEvents("onmouseover=function() { testFunc(); }");
    menuObj.addEvents("onmouseout=function() { testFunc(); }");

    Secondly you could try:
    menuObj.onmouseover=function() { testFunc(); }
    menuObj.onmouseout=function() { testFunc(); }

    Again, I can't stress enough how much of a beginner I am, but I think
    it's worth a try.

    I think the second one is most likely to work, but I can't be sure.
    Please note, I have enclosed the function you wish to use within a
    function(){} statement. I believe that if this isn't done, then the
    function will be called as the script is loaded, and not actually
    assigned to the event.

    Best of luck.

    Daz, Nov 17, 2006
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  3. Ron Goral

    Ron Goral Guest

    I appreciate the suggestion. The idea of the addAttributes and addEvents
    functions is to allow a person to define the menu in an HTML page without
    accessing the .js file (where this will finally reside).
    This ^ does not work.
    This ^ does work.
    Unfortunately, when the object code resides in a .js file, this is not going
    to be feasible as I want the HTML author to be able to define the event in a
    This does indeed happen.

    Thank you for the suggestions. I am further along the road of understanding,
    however, I am certain there must be a way to do this. I can't be the first
    or only person to ever want to do this sort of thing. Am I? ;P

    Peace -
    Ron Goral, Nov 17, 2006
  4. Ron Goral

    Ron Goral Guest

    eval() is the key. =)
    The code considered the arguments being passed as strings. Of course, a
    string cannot be executed, unless you use eval().

    Using these modified calls:

    menuObj.addEvents("event","onmouseover=function() { testFunc(); }");
    menuObj.addEvents("event","onmouseout=function() { testFunc(); }");

    And (greatly) modifying the addProperty function to be this:

    function addProperty(propertyName,keyValue)
    var type = (propertyName.indexOf("=") >= 0)?'standalone':propertyName;
    for (var i=0;i<addProperty.arguments.length;i++)
    var arg = addProperty.arguments.split("=");
    // something like innerHTML
    if (type == 'standalone'){this.element[arg[0]] = arg[1];}
    // an event property
    else if (type == 'event'){this.element[arg[0]] = eval("function(){"
    + arg[1] + "}");}
    // style and other named attributes
    else {this.element[addProperty.arguments[0]][arg[0]] = arg[1];}

    Produces the desired results. =)

    Thanks for the help Daz.
    Peace -
    Ron Goral, Nov 17, 2006
  5. Ron Goral

    Ron Goral Guest


    this.element[arg[0]] = eval("function(){"+arg[1]+"}");

    Should be this:

    this.element[arg[0]] = function(){eval(arg[1]);};
    Ron Goral, Nov 17, 2006
  6. Ron Goral

    Daz Guest

    Yes indeed. Does that mean it's working now?
    Daz, Nov 18, 2006
  7. Ron Goral

    Daz Guest

    Glad to have helped. Well done for figuring it out. :D
    Daz, Nov 18, 2006
  8. I wish to share this routine (for which I must lookup the source...)

    function addEvent(obj, evType, fn){
    var res;
    if (obj.addEventListener) {
    obj.addEventListener(evType, fn, false);
    res = true;
    } else if (obj.attachEvent) {
    var r = obj.attachEvent("on"+evType, fn);
    res = r;
    } else {
    res = false;
    return res;

    obj is an HTMLElement; evType is a string like "click"; fn is a function

    Let me elaborate on that last remark. This is a function definition:

    function doWork(arg) {

    This is a corresponding function call:


    and this is the function reference:


    If you want to assign any function to a property like onclick, you must
    assign the reference to the property:

    thatDiv.onclick = doWork;

    Alas, you cannot pass parameters to the function this way. Built-in
    behaviour, nonetheless, is for the browser to pass the current event to
    the function. Any event handling function should accept an event object
    (unless you absolutely want to ignore the identity of the event, or do
    not need its information). There is a little cross-browserity on it.
    Microsofts browser does not pass the event, but has a global variable
    'event' which carries the current event.

    Lumping this together, your event handler function could look like

    function doWork(e) {
    if (!e) e = window.event;
    // now you can be sure that e points to the current event object

    // do whatever is necessary here

    If you would put this:

    thatDiv.onclick = doWork(argument);

    doWork would be evaluated/executed, and its *result* is assigned to
    onclick. Needless to say this will produce no effect when you actually
    would click the element...

    I have one other remark. When I create objects (classes, I like to call
    them), I always include a private variable 'self' that is a copy of
    'this'. It saves a lot of confusion as to what exactly this refers to,
    when you are inside an object's functions.

    function myObject() {
    var self = this;

    self.value = 0; // public member

    // not really necessary here:
    self.someMethod = functionname;

    // nor here:
    self.otherMethod = function(arg) {
    // function body
    self.value = 23; // 'this' would point to the function otherMethod!

    This style may save you several "this.method is not a function" errors.

    Ron Goral schreef:
    Bas Cost Budde, Nov 18, 2006
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