wait-for.html

<script>
  /**
   * Wait for a condition to be true before continuing.
   * @param {Number} [delay] a timeout (in ms) for when an error should be thrown for a function that will never be true.
   *   Defaults to `2000`.
   * @param {Function<Boolean>} fn the function to poll to determine if the program should continue.
   *   Return `true` when the Promise should resolve.
   * @return {Promise} resolves when `fn` returns `true`.  Rejects with an `EvalError` if `fn` doesn't return `true` before `delay`.
  */
  window.waitFor = function(delay, fn) {
    if(!fn) {
      fn = delay;
      delay = 2000;
    }
    const startTime = Date.now();
    const timeoutError = new EvalError("waitFor function never returned 'true'.");
    let responded = false;
    return new Promise((resolve, reject) => {
      let responded = false;
      let poll;
      function test() {
        try {
          if(fn() === true && responded === false) {
            responded = true;
            clearInterval(poll);
            resolve();
          }
        }
        catch (err) { return; }
      }
      poll = setInterval(test, 10);
      setTimeout(() => {
        if(responded) { return; }
        clearInterval(poll);
        responded = true;
        reject(timeoutError);
      }, delay);
    });
  }
</script>