JSFiddle - React, Tailwind, and code Playground

HTML

<body>
  <div class="modal fade" id="myModal" tabindex="-1" role="dialog">
      <div class="modal-dialog">
        <div class="modal-content">
          <div class="modal-header">
            <button type="button" class="close" data-dismiss="modal" aria-label="Close"><span aria-hidden="true">&times;</span></button>
            <div class="top-image">
              <img src="https://tutsplus.github.io/merry-christmas-web-app-build/images/xmas-tree.png" alt="" />
            </div>
            <h1 class="modal-title">I wish you</h4>
          </div>
          <div class="modal-body">
            <p>Merry Christmas and Happy Holidays!</p>
            <hr />
            <p>Its time to say "<span>Thank You!</span>"</p>
          </div>
          <div class="modal-footer">
            <div class="img-footer">
            </div>
          </div>
        </div><!-- /.modal-content -->
      </div><!-- /.modal-dialog -->
    </div><!-- /.modal -->
    </body>

JavaScript

var particleCount = 300;
var particleMax   = 1000;
var sky           = document.querySelector('.modal');
var canvas        = document.createElement('canvas');
var ctx           = canvas.getContext('2d');
var width         = sky.clientWidth; //THIS LINE
var height        = sky.clientHeight;
var i             = 0;
var active        = false;
var snowflakes    = [];
var snowflake;

console.log(sky)