js onsubmit prevent default

<form onsubmit="return mySubmitFunction(event)">
  ...
</form>

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

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