JavaScript waitFor Polling

By  on  

As more of the JavaScript developers write becomes asynchronous, it's only natural to need to wait for conditions to be met. This is especially true in a world with asynchronous testing of conditions which don't provide an explicit await. I've written about waitForever, waitForTime, and JavaScript Polling in the past, but I wanted to have a more modern way of awaiting a given state. Let's have a look at this super useful waitFor function!

waitFor is an async function that allows developers to provide a condition function, polling interval (in milliseconds), and optional timeout (in milliseconds).

// Polls every 50 milliseconds for a given condition
const waitFor = async (condition, pollInterval = 50, timeoutAfter) => {
  // Track the start time for timeout purposes
  const startTime = Date.now();

  while (true) {
    // Check for timeout, bail if too much time passed
    if(typeof(timeoutAfter) === 'number' && Date.now() > startTime + timeoutAfter) {
      throw 'Condition not met before timeout';
    }

    // Check for conditon immediately
    const result = await condition();

    // If the condition is met...
    if(result) {
      // Return the result....
      return result;
    }

    // Otherwise wait and check after pollInterval
    await new Promise(r => setTimeout(r, pollInterval));
  }
};

Using this function is as simple as just providing a condition function:

await waitFor(() => document.body.classList.has('loaded'));

Timing out the interval and timeout is also simple:

await waitFor(
  () => document.body.classList.has('loaded'),
  // Checks every 100 milliseconds
  100,
  // Throws if the "loaded" class isn't on the body after 1 second
  10000
);

In an ideal world, developers would always have a handle on the Promise that could be await'd or then'd. In practice, however, that isn't always the case, especially in a testing environment. Being able to await a condition in any environment is an absolute must, so keep this snippet in your toolbox!

Recent Features

  • By
    Responsive Images: The Ultimate Guide

    Chances are that any Web designers using our Ghostlab browser testing app, which allows seamless testing across all devices simultaneously, will have worked with responsive design in some shape or form. And as today's websites and devices become ever more varied, a plethora of responsive images...

  • By
    5 Awesome New Mozilla Technologies You’ve Never Heard Of

    My trip to Mozilla Summit 2013 was incredible.  I've spent so much time focusing on my project that I had lost sight of all of the great work Mozillians were putting out.  MozSummit provided the perfect reminder of how brilliant my colleagues are and how much...

Incredible Demos

  • By
    Web Audio API

    The Web Audio API allows developers to load and decode audio on demand using JavaScript.  The more I evaluate awesome games for Firefox OS TVs, the more I get to learn about these APIs that I normally wouldn't touch.  The following is a very basic introduction to the WebAudio API...

  • By
    Add Site Screenshots for External Links Using MooTools Tooltips

    Before you send your user to an unknown external website, why not provide them a screenshot of the site via a tooltip so they may preview the upcoming page? Here's how you can do just that using MooTools. The MooTools JavaScript The first step is to grab...

Discussion

  1. Who knew asynchronous programming could be this elegant? This waitFor tutorial is a game-changer for smoother code execution. Thanks for sharing, David Walsh!

Wrap your code in <pre class="{language}"></pre> tags, link to a GitHub gist, JSFiddle fiddle, or CodePen pen to embed!