Here is a simple jQuery script which could use the validate a mandatory input.

(function ($) {
  $(document).ready(function() {    
    $('#<submit-button-id>').click(function(event){    
      var x = $('<input-id>').val();
      if (x == null || x == '';) {
        alert('Please fill in the input.');
        event.preventDefault();
      }
    })
  });
})(jQuery);

Done =)

Reference: StackOverflow – Intercept a click on submit button and check validity of form jQuery