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What is the proper way to get the element ID for a client script?

For example, suppose in the WebControl:
TextBox t=new TextBox;
t.ID=this.ClientID+"X";
....
Then elsewhere we want to access it on the client:
Control.Attributes["OnClick"] =
"javascript:document.getElementById('"+this.Client ID+"X"+"').value=
'test';";

It doesn't work because the textbox ID becomes something like:

TextBox1_X

and the link control looks like this:

javascript:document.getElementById('TextBox1X').va lue= 'test';