Retrieving control using javascript given a control's id

I am trying to do this right now using multiple user controls on a page. The user control is a file browser control that was written which contains 5 objects, 2 server asp controls and 3 client inputs. What I'm trying to do is get the main page to locate one of the two server controls within the user control for the input that was clicked on.

...
<asp:TextBox ID="Textbox1" runat="server"/>
<input type="button" id="button1"/>
...

in main page

var someValue = '';

function setValueInUC(someValue){
   var sTxtName = 'txt' + someValue;
   document.getElementById(sTxtName).value = someValue;
}