using javascript to change .net web form properties

Hello everyone, I'm fairly new to ASP.NET and I'm trying to find the best solution to this problem I'm having.  I believe ultimately what I need to be able to do is change the ImageUrl value via javascript of an ImageButton.  A small example of what I'm working on can be found here in a javascript form:

This is a simple data input method for a survey. There are three distinct columns, when you select a number within a column it will change the .gif image to show it's selected and then populate the textbox with that value.  If you change the value of the textbox manually it will change the .gif according to your selection.  This is the javascript version.  If you look at the source you will see the images (numbers) as this:

<a href=# onClick="s1a_1_fqy_One_BC()";><img name="s1a_1_fqy_One" border="0" src="images/one.gif"></a>

the onClick calls a javascript function that changes the image src.  The text input boxes look like this:

<input type="text" onChange="s1a_1_fqy_TC()"; name="s1a_1_fqy">

the onChange will check which value is in the field and then "select" the corresponding .gif image. 

This javascript solution works great except for the fact that I need to resort to standard html web forms and sacrifice the .net web forms.

Initially I had written a non-javascript version using ImageButtons and and asp textboxes, the problem was, each time something was selected the page would have to postback to update it.  This is extremely annoying and hard to work with considering there are several rows on this survey.  Even when I set the smart navigation directive to true it was difficult to work with.  This is most likely do to the fact that the code behind file was like 180k.   

I think ultimately what I need is to be able to change the ImageUrl of the ImageButton item via javascript, but I'm not sure if that's even possible, or how to go about doing it.

Any opinions or advice would be greatly appreciated! Thanks.

6/1/2005 1:37:58 PM 24353 articles. 1 followers. Follow

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Hi Matt,

   I'm a little confused.  You say you need to get rid of the part completely, but you're still referring to ImageUrls and ImageButtons; which is it?  Also, I noticed that you're putting semicolons *outside* your event handler assignments; this isn't necessary or standard and could confuse some browsers.

   For what it's worth, I'd suggest consolidating your JS functions rather than having one for each image and textbox.  You can pass parameters to the function to tell it which images to roll and whatnot.  Something like this might work:  assume you have txtA, txtB, and txtC, and 4 image tags for each (imgA1, imgA2, etc).  You'd probably want to rename the image files (e.g. imgA1.gif, imgA1_sel.gif) to make this a bit simpler, and then for each textbox, you could assign an event handler like this:

<input name="txtA" onchange="changeAnswer(this, 'A')" />

On each image tag (or link), you could change the onclick to look more like this:
<img name="imgA1" onclick="changeAnswer(this, 'A', 1)" />

Then your function would look something like this:

function changeAnswer(theField, theCategory, theNumber) {
   if("txt") == 0) { // This is a text field based on our naming convention
      // Get an integer from the field that we can use
      theNumber = parseInt(theField.value); // (since this isn't passed to us)
      if(!(theNumber > 0 && theNumber < 5)) { alert("Out of range");; return; }
   else { // Somebody clicked an image (you could put other cases in here as well)
      document.controlForm["txt" + theCategory].value = theNumber;

   for(var i=1;i<=4;i++) { // Arrange all the images correctly
      if(i==theNumber ) {
         document.controlForm["img" + theCategory + theNumber ].src = "img" + theNumber + "_sel.gif";
      else {
         document.controlForm["img" + theCategory + theNumber ].src = "img" + theNumber + "_sel.gif";


   I haven't tested this, and I know there are more efficient ways to do some of this, but since you're just starting out, I figured this would be enough for now.
   Yell if you have questions.



Peter Brunone
Do the impossible, and go home early.
6/1/2005 2:53:13 PM
Thanks for the response and the tips.  I actually do not wish to
get rid of the .net web forms.  However, there are only two ways I
know how to do this:

1. with strictly using .net web forms
2. using javascript and regular HTML web forms.  (basically resorting to classic asp is what this would be)

What I really want though is:

3. Keeping the .net web forms and using javascript within them. 

The issue with the 1st option is that the files are too large - and this is really due to my amateurish programming experience (as you saw).  Each time it does a post back, it has to reload a huge file, like 180k.  The 2nd option has the ideal usability since the scripting is client side, but I only know how to do it with standard HTML forms.  

I know I really need to work on making my code more efficient first, and the whole concept of using javascript within dot net web forms is really fuzzy to me.  For example, I'm not sure of the possibilities and limitations.  One example would be, is it possible to use javascript to change the ImageUrl="" value within and ImageButton via an onClick?


6/1/2005 4:05:50 PM

   First of all, remember that you don't *have* to use WebControls in an aspx form.  You can just use regular HTML tags if you like, or add runat="server" if you need them in Viewstate and such.

   Second, you can add javascript event handling code to most controls by putting into the Attributes collection, like so:

myTextBox.Attributes.Add("onchange", "changeAnswer(this, 'A')")

   Don't forget to set autopostback to false where applicable.  For your images, if the only function they have is client-side changes, then I'd say don't use ImageButtons at all.

   Keep the questions coming...

Peter Brunone
Do the impossible, and go home early.
6/1/2005 4:35:23 PM
 I've pretty much concluded that the ImageButtons and ASP
TextBoxes were uncessary.  However, I'm running into a problem
where I'm have a aspx page like this:


<form runat=server id="controlForm">
-- necessary ImageButtons and TextBoxes
-- regular textboxes and images with javascript function calls
-- more necessary ImageButtons and TextBoxes

When these javascript functions get called, and they try to reference the form or an object such as this:

document.images["img" + theCategory + theNumber].src = "images/img" + theNumber + "_sel.gif";
document.controlForm["txt" + theCategory].value = theNumber;

I get document.controlForm is null or not an object.  I know this is due to the aspx form versus the regular html forms, and I've tried including two which of course does not work.  Any ideas?
6/2/2005 1:28:21 AM
When you see null or not an object that means one of two things.

The element does not exist on the page or you mispelt something.This is solved by fixing the spelling or adding the element.


You are calling the function before the page has been loaded? This can be solved by using an onload handler.

Eric | Twitter epascarello | LinkedIn
6/2/2005 2:14:58 AM
View Source in the browser and see what name the form tag has.  The name attribute is what you need to use document.formName.

You can also get to elements directly if you assign an ID attribute, with document.getElementById("elementID").


Peter Brunone
Do the impossible, and go home early.
6/2/2005 2:16:44 AM
Looks like the form in that sample page is "controlform", whereas you're calling "controlForm".  Javascript is case-sensitive, so those will bite you now and then.

Peter Brunone
Do the impossible, and go home early.
6/2/2005 2:19:10 AM

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