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.

  The callback's responsibility is to provide the value when the promise is fulfilled.
  It can be used to return classes or objects created by code from deferred script tags.

  ~Bu

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