You can bind an event handler to the "submit" JavaScript event using jQuery .submit
method and then get trimmed #log
text field value and check if empty of not like:
$('form').submit(function () {
// Get the Login Name value and trim it
var name = $.trim($('#log').val());
// Check if empty of not
if (name === '') {
alert('Text-field is empty.');
return false;
}
});