Find Asp.net Web Forms control in script in Js (JavaScript) file

The issue of course is that you wishing to have a hard coded text box name, and then use a external .js file. External files don't get nor enjoy server side expression processing.

There are two approaches here:

Use clientidMode="static" for the text box.

Hence this:

        <asp:TextBox ID="TextBox1" runat="server" ClientIDMode="Static" >
        </asp:TextBox>

And now your js code becomes this:

var obj = document.getElementById("TextBox1");

Another approach?

Place BOTH the control and JavaScript inside of a user control.

So, say this:

<asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>

<script>
    function mytest() {
        var tBox = document.getElementById('<%=TextBox1.ClientID%>')
        alert(tBox.value)
    }

</script>

Now, our markup becomes this:

        <asp:Button ID="Button1" runat="server" Text="Show text box value"
            OnClientClick="mytest();return false"
            />
             <br />

        <uc1:MyScriptControl runat="server" id="MyScriptControl" />

And the result is this:

You can also of course place some small js code stubs in the current page that calls/uses/enjoys the external .js library of code, but you thus have to "pass" the control value with smaller code stubs in the current page.

So, say this:

   <script>

        function mytestlocal() {
            var obj = document.getElementById("<%= TextBox1.ClientID %>");
            mytest(obj)
        }
   </script>

And thus in the external file we have this:

  function mytest(tBox) {
    alert(tBox.value)
}

So, the "very" idea of a external .js file means that external file can't have hard code controls, nor can it use or have <%=%> (server side) expressions.