elementExistPromise.js

const elementExistPromise = (selectorFunc, parent = document.body) => {
  const exists = selectorFunc(parent);
  if (exists && exists.length !== 0) {
    return Promise.resolve(exists);
  }
  return new Promise(resolve => {
    const mu = new MutationObserver((list, me) => {
      const record = list.find(({ target }) => {
        const x = selectorFunc(target);
        return x && x.length !== 0;
      });
      if (record) {
        me.disconnect();
        resolve(selectorFunc(record.target));
      }
    });
    mu.observe(parent, {
      childList: true,
      subTree: true,
      attributes: true,
    });
  });
};

const findNode = (fn, node) => {
  if(fn(node)) {
  	return node;
  }
  for (let i = 0; i < node.children.length; i ++) {
  	const child = findNode(fn, node.children.item(i))
    if (child) {
    	return child
    }
  }
  return null;
}