Master Pages and JavaScript document.getElementById

using System;
using System.Data;
using System.Configuration;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using System.Text;

public partial class _Default : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        // Here we need to register the client ids to the client, so that the same can be used in the javascript
        // If there are nultiple controls make it by comma seperated..
        RenderJSArrayWithCliendIds(txtTest);
    }

    // This is the method used to register the array of the clientid's as well as the serverid's
    // Also this method registers the function GetClientId, which is used to get the client id provided server id is supplied
    public void RenderJSArrayWithCliendIds(params Control[] wc)
    {
        if (wc.Length > 0)
        {
            StringBuilder arrClientIDValue = new StringBuilder();
            StringBuilder arrServerIDValue = new StringBuilder();

            // Get a ClientScriptManager reference from the Page class.
            ClientScriptManager cs = Page.ClientScript;

            // Now loop through the controls and build the client and server id's
            for (int i = 0; i < wc.Length; i++)
            {
                arrClientIDValue.Append("\"" + wc[i].ClientID + "\",");
                arrServerIDValue.Append("\"" + wc[i].ID + "\",");
            }
            // Register the array of client and server id to the client
            cs.RegisterArrayDeclaration("MyClientID", arrClientIDValue.ToString().Remove(arrClientIDValue.ToString().Length - 1, 1));
            cs.RegisterArrayDeclaration("MyServerID", arrServerIDValue.ToString().Remove(arrServerIDValue.ToString().Length - 1, 1));
            // Now register the method GetClientId, used to get the client id of tthe control
            cs.RegisterStartupScript(this.Page.GetType(), "key", "\nfunction GetClientId(serverId)\n{\nfor(i=0; i<MyServerID.length; i++)\n{\nif ( MyServerID[i] == serverId )\n{\nreturn MyClientID[i];\nbreak;\n}\n}\n}", true);            
        }
    }
}