Classic website - JavaScript usage

format.js

import $ from 'jquery';

const format = () => {
  $('.btn').each((i, e) => {
    const that = $(e);
    that.addClass('btn-danger');
  });
};

export default format;

index.html

...
<script src="//code.jquery.com/jquery-2.2.4.min.js" crossorigin="anonymous"></script>
<script src="/build/js/bundle.js"></script>
...

index.js

import $ from 'jquery';

import format from './format';
import { toggleColor, toggleVisible } from './toggle';

$(document).on('ready', () => {
  format();
  toggleColor();
  toggleVisible();
});

toggle.js

import $ from 'jquery';

export const toggleColor = () => {
  if ($('#my-button').length > 0) {
    $('#my-button').on('click', e => {
      const that = $(e.currentTarget);
      that.next().toggleClass('text-danger text-success');
    });
  }
};

export const toggleVisible = () => {
  if ($('#my-button').length > 0) {
    $('#my-button').on('click', e => {
      const that = $(e.currentTarget);
      that.next().toggleClass('hidden');
    });
  }
};