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();});

@acropup, an alternative solution is to match on the unique ID only, and not use selectors. This would be intuitive, since the promise resolves after the first match is found.

However, if there was a way to reliably tokenise a selector, it could be found through introspection what kind combinators are present and adjust the match/query algorithm accordingly. There is a css-selector-tokenizer package, but that seems overkill for something that should be easily achievable.

section:has(div > strong) example using closest(). Adjacent and sibling combinators can also be implemented, but the important aspect in terms of performance is having a tokeniser to introspect and recognise which algorithm to pick based on the complexity of the selector. Although even without that, this should represent a substantial improvement.

const element = (addedNode.matches(selector) && addedNode)
  || addedNode.querySelector(selector)
  || addedNode.closest(selector);

ok boo