[WIP] An RTL-like `waitFor` utility, without the DOM stuff

waitFor.js

export default async function waitFor(callback, { timeout = 1000 } = {}) {
  let intervalTimer;
  let lastError = null;
  let pending = false;
  let timeoutTimer;

  return new Promise((resolve, reject) => {
    function onDone(result) {
      clearInterval(intervalTimer);
      clearTimeout(timeoutTimer);

      pending = false;

      resolve(result);
    }

    function onError(error) {
      pending = false;
      lastError = error;
    }

    timeoutTimer = setTimeout(() => {
      clearInterval(intervalTimer);
      reject(lastError);
    }, timeout);

    intervalTimer = setInterval(() => {
      if (pending) return;

      try {
        const result = callback();

        if (result.then) {
          pending = true;

          result.then(onDone, onError);
        } else {
          onDone(result);
        }
      } catch (error) {
        onError(error);
      }
    });
  });
}