mencegah pengiriman formulir pada panggilan fungsi onsubmit

returning false here won't be executed and the form will be submitted either way. You should also call preventDefault to prevent the default form action for Ajax form submissions.

function mySubmitFunction(e) {
  e.preventDefault();
  someBug();
  return false;
}

Get more references : https://stackoverflow.com/questions/3350247/how-to-prevent-form-from-being-submitted
Ankur