How to call external js file from content page?

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C#

ASP.NET

my Java script code in .js file is

function fillTextBox() {

               var dropdown = c.selectedIndex;alert("fdgdfg");
               var selvalue = c[dropdown].value;
               if(selvalue=="True")
               {
              v.value=1;
               }
               else
               v.value=0;
           }

I assigned my web controls to variables in aspx page
<script type="text/javascript" src="../../JavaScript/UserFunctions.js"></script>

<script type="text/javascript" language="javascript">
var c= document.getElementById('<%= ddlCompanyChit.ClientID %>');
var v=document.getElementById('<%= txtTcktNum.ClientID %>')
</script>

and am calling the function from code behind

ddlCompanyChit.Attributes.Add("OnChange", "fillTextBox()");

I think the Problem is .js file could not find the web controls..so please provide the solution for how to pass web controls to .js file?