Reference ASP.NET control by ID in JavaScript?

This post by Dave Ward might have what you're looking for:

http://encosia.com/2007/08/08/robust-aspnet-control-referencing-in-javascript/

Excerpt from article:

Indeed there is. The better solution is to use inline ASP.NET code to inject the control’s ClientID property:

$get('<%= TextBox1.ClientID %>')

Now the correct client element ID is referenced, regardless of the structure of the page and the nesting level of the control. In my opinion, the very slight performance cost of this method is well worth it to make your client scripting more resilient to change.

And some sample code by Dave from the comment thread of that post:

<script>
  alert('TextBox1 has a value of: ' + $get('<%= TextBox1.ClientID %>').value);
</script>

The comment thread to the article I linked above has some good discussion as well.

Couple of thoughts on this:

1) I've had a lot of luck getting elements by css class instead of id because asp.net ids are not reliable as you stated. I use this function and it performs reasonably well:

function getElementsByClass(searchClass,node,tag) {
 var classElements = new Array();
 if ( node == null )
    {
        node = document;
    }

 if ( tag == null )
    {
        tag = '*';
    }

 var els = node.getElementsByTagName(tag);
 var elsLen = els.length;
 var pattern = new RegExp("(^|\\s)"+searchClass+"(\\s|$)");

 for (i = 0, j = 0; i < elsLen; i++) 
    {
        if ( pattern.test(els[i].className) ) 
            {
                classElements[j] = els[i];
                j++;
            }
      }
 return classElements;
}

2) jQuery helps here alot. Using jQuery you can reliably get elements where the id ends with a certain string. While this is not "the" reason to use jQuery it's definitely a plus.

3) This will be fixed in asp.net 4.0 so hang in there :-) http://weblogs.asp.net/asptest/archive/2009/01/06/asp-net-4-0-clientid-overview.aspx

I don't think there's a single "best practice" for doing this. There's plenty of different pretty good practices. Here's ours:

Every control which has client-side functionality renders a script block inline, directly below the markup for the control:

<span id="something_crazy_long">
    control markup
</span>
<script type="text/javascript">new CustomControl('something_crazy_long');</script>

Each control has an accompanying JS like:

var CustomControl = function(id) {
    this.control = document.getElementByID(id);
    this.init();
};

CustomControl.prototype.init = function() {
    //do stuff to wire up relevant events
};

In the codebehind, we do something like:

class CustomControl : Control

override void Render(HtmlTextWriter writer)
{
    writer.WriteBeginTag("span");
    writer.WriteAttribute("id", this.ClientID);
    writer.Write(HtmlTextWriter.TagRightChar);
    //write control markup
    writer.WriteEndTag("span");
    writer.WriteBeginTag("script");
    writer.WriteAttribute("type", "text/javascript");
    writer.Write(HtmlTextWriter.TagRightChar);
    writer.Write(
        string.Format("new CustomControl('{0}');", this.ClientID)
    );
    writer.WriteEndTag("script");
}

I do something similar to Rex M except to avoid multiple script tags I use a function in my page base class to register the controls I am going to use clientside, then spit them out to html inside 1 script tag.

You could even subclass your controls to automatically register with the function or use a boolean property to set whether you are going to use them clientside.

Oh, and I also found this, in case anyone else is having this problem.

Use a custom jQuery selector for asp.net controls: http://john-sheehan.com/blog/custom-jquery-selector-for-aspnet-webforms/