Net list id in javascript on client.

Question

I need to get the ID of the dropdown (ASP.Net Control) so that I can determine if an item has been selected.

I am currently trying to simply write a dropdown counter in the alert box like this:

OnClientClick="alert(document.getElementID('<%=ListBox1.ClientID %>').options.length)

The error I am getting is "required document".

+1

Eric May 14 '09 at 17:41

3 answers

Guffa · Answer 1 · 2009-05-14T17:49:08+0000

First, adjust getElementID

in your code to getElementById

:

OnClientClick="alert(document.getElementById('<%=ListBox1.ClientID %>').options.length);"

If you want to know which item is selected use the property selectedIndex

:

OnClientClick="alert(document.getElementById('<%=ListBox1.ClientID %>').selectedIndex);"

If you want a parameter value rather than an index, use the indexed options collection:

OnClientClick="var s=document.getElementById('<%=ListBox1.ClientID %>');alert(s.options[s.selectedIndex].value);"

Edit:
This will work if the control you are trying to use it in is not server-side, e.g .:

<input type="button" onclick="alert(document.getElementById('<%=ListBox1.ClientID %>').options.length);" />

Since you have a server control, you cannot use the script (<% =%>) tag inside the control. You have to set the property from code:

TheButton.OnClientClick = "alert(document.getElementById('" + ListBox1.ClientID + "').options.length);";

Vikram · Answer 2 · 2009-05-14T17:46:24+0000

To use declaration document.getElementById instead of document.getElementId

Paul U · Answer 3 · 2009-05-14T23:12:29+0000

My javascript is a little rusty, but can't you use the "this" keyword? sort of:

OnClientClick="alert(this.options.length);"