Inserting Javascript into ASP.Net page

I have the code

<script type="text/javascript">
    function validateLength(objectSource, args) {
        var a = document.getElementById('<%=txtUserName.ClientID%>').value;
        args.IsValid = (a.length >= 5 && a.length <= 10);
    }
</script>

Which I have in my ASP.NET page.

This works fines. Will validate a textbox to make sure it is between a certain length.

However I am trying to tidy up my page's code. So decided to put this little function into a separate javascript file.

<script type="text/javascript" src="Scripts/MyJS.js">
</script>

Page loads fine and then when the validation comes around it fails. I am guessing it cannot access the textbox( txtUserName).

My question is: How can I get this to work?

I get the error message

0x800a138f - Microsoft JScript runtime error: Unable to get value of the property 'value': object is null or undefined

when the javascript fires

When you include a javascript file, the part of code

<%=txtUserName.ClientID%>

isn't rendered.

You have to change the javascript function in two ways Fixing the name of the control

<script type="text/javascript">
    function validateLength(objectSource, args) {
        var a = document.getElementById('txtUserName').value;
        args.IsValid = (a.length >= 5 && a.length <= 10);
    }
</script>

or adding a parameter

<script type="text/javascript">
    function validateLength(objectSource, args, nameOfControl) {
        var a = document.getElementById(nameOfControl).value;
        args.IsValid = (a.length >= 5 && a.length <= 10);
    }
</script>

then you can call the function from your page and the parameter nameOfControl could be

<%=txtUserName.ClientID%>

for example

validateLength(objectSource, args, <%=txtUserName.ClientID%>)