Wait for an element to exist. ES6, Promise, MutationObserver

es6-element-ready.js

// MIT Licensed
// Author: jwilson8767

/**
 * Waits for an element satisfying selector to exist, then resolves promise with the element.
 * Useful for resolving race conditions.
 *
 * @param selector
 * @returns {Promise}
 */
export function elementReady(selector) {
  return new Promise((resolve, reject) => {
    let el = document.querySelector(selector);
    if (el) {
      resolve(el); 
      return
    }
    new MutationObserver((mutationRecords, observer) => {
      // Query for elements matching the specified selector
      Array.from(document.querySelectorAll(selector)).forEach((element) => {
        resolve(element);
        //Once we have resolved we don't need the observer anymore.
        observer.disconnect();
      });
    })
      .observe(document.documentElement, {
        childList: true,
        subtree: true
      });
  });
}

example.js

import { elementReady } from "es6-element-ready";
// Simple usage to delete an element if/when it exists:
elementReady('#someWidget').then((someWidget)=>{someWidget.remove();});

Thanks @jwilson8767 for the last script,
it did work for me. also i m not sure about if it s more efficient than using mutationobserver..

Crazy hard to understand this script though, quite complicate.

i personaly removed the async and did a return Promise.resolve(el); for the first return.
also added a settimeout to test if the script run more than 4second to stop it in case it didnt find all matches.

@bezborodow Regarding performance issues, elementReady() should definitely be taken as a convenience function, I still use it occasionally but only where it is expected to run only a few times in the lifecycle of a page (such as to detect when a component's root element has been added to the page). MutationObservers as a whole aren't amazing performance-wise, really. For much better performance (while still working on arbitrary selectors), I'd actually recommend using a sort of long-polling strategy via setInterval. You could either create one setInterval per selector, or a shared setInterval and an array of outstanding selectors to match against. Here's an example of the latter:

// dict of {selector: [promise, resolve]}
const pendingElements = {};
let pendingElementsInterval;
const elementReadyBatchedFrequency = 100; // ms

/**
 * Wait for an element to be ready using a querySelector
 *
 * @param selector {string}
 * @param containerEl {Element} optional container element to search within
 * @returns {Promise<Element>}
 */
async function elementReadyBatched(selector, containerEl = document){
    let el = document.querySelector(selector);
    if (el) {
      return el;
    }
    // group outstanding requests
    if (pendingElements[selector]) {
      const [promise, _] = pendingElements[selector];
      delete pendingElements[selector];
      return promise;
    }
    let resolve;
    const promise = new Promise(r => resolve = r);
    pendingElements[selector] = [promise, resolve];
    if (!pendingElementsInterval) {
      pendingElementsInterval = setInterval(() => {
        for (const [_selector, [_, _resolve]] of Object.entries(pendingElements)) {
          const el = containerEl.querySelector(_selector);
          if (el) {
            _resolve(el);
            delete pendingElements[_selector];
          }
        }
        if (!Object.keys(pendingElements).length) {
          clearInterval(pendingElementsInterval);
          pendingElementsInterval = null;
        }
      }, elementReadyBatchedFrequency);
    }
    return pendingElements[selector][0];
}

edit: yes, I know this is very late in coming. Still <3 you.