Javascript error log says function not defined when I have defined it in a separate file and even the same file

This is the smallest simplest start I could think of, and of course it errors. I'm running a .cshtml page. Here is the code for having function in the SAME file just lines apart:

@section head
{
    <script>
        function CheckPostalCode(form) {
            if (form.postalcode.value.length < 5) {
                alert("Please enter a Zip Code.");
                form.postalcode.focus();
                return false;

            }

            return (true);
        }
    </script>
}

Now in the webpage below:

<form onsubmit="return CheckPostalCode(this)" method="post" action="/search/index.cshtml">
    <input type="text" name="postalcode" id="postalcode" size="30" maxlength="5" required/>
    <input type="submit" name="submit" value="go" />

</form>

The original method I tried to use was having a separte file called checkFields.js and including the file in the pages section head as such:

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

Where the webpage code is the same. Can anyone help explain why even when the script is in the same file its undefined. But really I would like to be able to call the function from another file. I've seen many tutorials that all say, you can call functions like normal so long as you include them. Well I thought I did.