Reference ASP.NET control by ID in JavaScript?

When ASP.NET controls are rendered their ids sometimes change, like if they are in a naming container. Button1 may actually have an id of ctl00_ContentMain_Button1 when it is rendered, for example.

I know that you can write your JavaScript as strings in your .cs file, get the control's clientID and inject the script into your page using clientscript, but is there a way that you can reference a control directly from JavaScript using ASP.NET Ajax?

I have found that writing a function to parse the dom recursively and find a control that CONTAINS the id that I want is unreliable, so I was looking for a best practice rather than a work-around.

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.

You can change to ClientIDMode property of the control to 'Static' that will result the same ID that you give the control in the .NET code.

<asp:TextBox ID="TextBox1" ClientIDMode="Static" runat="server"></asp:TextBox>

will result:

<input name="ctl00$MainContent$TextBox1" type="text" id="TextBox1">

so you have the same ID.

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

Reference ASP.NET control by ID in JavaScript?

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