JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://code.jquery.com/jquery-3.1.0.min.js"></script>
<form>
  <input type="text" name="a" value="" required>
  <input type="text" name="b" value="" required>
  <input type="submit" id="submit_button" disabled>
</form>

CSS

#submit_button {
  display: none;
}

#submit_button.active {
  display: inline;
}

JavaScript

var submit = $('#submit_button'),
    required = $('input[required]');

required.on('keyup', function() {
  if (0 == required.filter(function() { return '' == this.value; }).length) {
    submit.addClass('active').prop('disabled', false);
  } else {
    submit.removeClass('active').prop('disabled', true);
  }
});