[Tampermonkey] waitForElement function

wait-for-element.js

function waitForElement(selector, fallbackSelector) {
  "use strict";

  return new Promise((resolve, reject) => {
    if (document.querySelector(selector)) {
      return resolve(document.querySelector(selector));
    }

    const observer = new MutationObserver((mutations) => {
      if (document.querySelector(selector)) {
        resolve(document.querySelector(selector));
        observer.disconnect();
      } else if (fallbackSelector && document.querySelector(fallbackSelector)) {
        reject("Fallback selector triggered.");
        observer.disconnect();
      }
    });

    observer.observe(document.body, {
      childList: true,
      subtree: true,
    });
  });
}