getting client id of a control from variable - asp.net

Question

I want the client id of an asp.net textbox control(txtTest) in javascript.But the problem here is the control id comes from a variable as shown below
var testName = 'txtTest';
var testCntrl = document.getElementById('<%=' + testName + '.ClientID %>');
But its throwing
CS1012: Too many characters in
character literal
Can any one please help....

Answer 1

Try this:
document.getElementById('<%=txtTest.ClientID %>');
Or more along the lines of your original example:
var testName = '<%=txtTest.ClientID %>';
var testCntrl = document.getElementById(testName);
It appears from your example that you have managed to confuse yourself over what is server side and what is client side code.

Answer 2

<%= aspControlID.ClientID %> is a server side control, but you are trying to pass a clientside variable name to it. By the time testName is set equal to 'txtTest' its too late, you're already on the client.
There are a number of alternatives to get the server side ClientIDs as Rick Stahl discusses.
1) You can pre-load all the control IDs that you know you're going to need like this, they query them (he uses jquery) when you need their elements.
var ids = {
txtSymbol: "#<%= txtSymbol.ClientID %>",
PageContent: "#<%= PageContainer.ClientID %>"
}
This can also be written:
var txtSymbol = document.getElementById('<%= txtSymbol.ClientID %>');
var txtBlah = document.getElementById('<%= txtBlah.ClientID %>');
2) Or, he wrote a function that will get a control for you from the clientside
function $$(id, context) {
var el = $("#" + id, context);
if (el.length < 1)
el = $("[id$=_" + id + "]", context);
return el;
}
Be aware that there are some serious caveats. This relies on JQuery, so be sure to include that library and use it like this $$('myASPControlID').val('new val'); The catch is that if you have any controls that create other controls, like listviews, repeaters, gridviews etc. Then finding a single instance of a child control will take some strategy. In that situation, this tool will only get the first instance of a repeated control.
Still, the function provides a way to solve this problem by allowing you to specify a containing element in the second field.
EDIT
Hey L G, if you really need to pass your variable from the client side, then just add the second function and a link to the JQuery library. Then you can get your control with this simple code:
var testName = 'txtTest';
var testCntrl = $$(testName);

Answer 3

If it is C# code (which I assume, given the compilation error), you need to surround strings with " instead of '.
' is used for char values, that can contain only one character (or two if it is an escaped one, such as '\n').
But I don't really get the connection between the compilation error and the code, since the C# compiler should not bother about the javascript code...