a Promise-based class that resolves when DOM has finished loading.

Deferred.js

/*

  A simple Promise-based class that will resolve only after the DOM has completely loaded.

  It can be used with or without a callback. But when a callback is supplied, the responsibility
  of resolving or rejecting the Promise is passed on to the callback.

  ~Bu

*/
class Deferred {
  constructor(callback) {
    const promise = new Promise((resolve, reject) => {
      const executor = () => {
        if (typeof callback === typeof (() => {})) {
          callback(resolve, reject);
        } else {
          resolve();
        }
      };
      const DOM_STATE_COMPLETE = 'complete';
      if (document.readyState === DOM_STATE_COMPLETE) {
        executor();
      } else if (document.addEventListener) {
        document.addEventListener('DOMContentLoaded', () => {
          executor();
        });
      } else {
        document.attachEvent('onreadystatechange', () => {
          if (document.readyState === DOM_STATE_COMPLETE) {
            executor();
          }
        });
      }
    });
    this.then = promise.then.bind(promise);
    this.catch = promise.catch.bind(promise);
    this.finally = promise.finally.bind(promise);
  }
}