Simple text field

The easiest way is to return the javascript function from the onSubmit event of your form. onSubmit = "return your_function()". That way, when your user clicks your submit button, your script can check whatever you want to check *before* the form is actually submitted.

Like this:

<script type="text/javascript" language="javascript">
function check_file_field()
{
var file_field = document.getElementById("flFileField");
if(file_field.value.length == 0)
{
alert("Select a file first!");
return false;
}
else
{
return true;
}
}
</script>

<cfform action="#cgi.script_name#" method="post" name="frmTest" preloader="no" onsubmit="return check_file_field()">
<cfinput type="file" id="flFileField" name="flFileField">
<cfinput type="submit" name="btnSubmit" value="Go">
</cfform>