Resize Observer

By  on  

Creating websites that are reactive and responsive used to be considered advanced but responsiveness is a necessity for successful websites and apps. We've added media queries, matchMedia, and a host of other APIs to help developers make responsiveness easier and now we get a new one: ResizeObserver. With the Resize Observer API, we can watch for resizing on individual elements!

Using ResizeObserver

To listen for resize changes on elements, create a ResizeObserver instance and call observe, passing an element:

const observer = new ResizeObserver(entries => {
  for (let entry of entries) {
    // Now do something with the resized element
    if (entry.contentRect.width < 1000) {
      // Stop making AJAX calls for content...
    }
  }
});
observer.observe(document.querySelector('div'));

An entry provides you a target element as well as its dimensions and positioning:

entry = {
  target: div, // The element passed to `observe`
  contentRect: {
  bottom: 88,
  height: 88,
  left: 0,
  right: 1043,
  top: 0,
  width: 1043,
  x: 0,
  y: 0
  }
}

Media queries and matchMedia provide an opportunity to adjust display via CSS but not functionality, which is where ResizeObserver fits in.

Years ago I created a hack for spying on elements using CSS, media queries, and :before, but it required polling via JavaScript to work properly. Having a legit, optimized JavaScript API to accomplish the same is refreshing and desperately needed!

Recent Features

  • By
    Conquering Impostor Syndrome

    Two years ago I documented my struggles with Imposter Syndrome and the response was immense.  I received messages of support and commiseration from new web developers, veteran engineers, and even persons of all experience levels in other professions.  I've even caught myself reading the post...

  • By
    How I Stopped WordPress Comment Spam

    I love almost every part of being a tech blogger:  learning, preaching, bantering, researching.  The one part about blogging that I absolutely loathe:  dealing with SPAM comments.  For the past two years, my blog has registered 8,000+ SPAM comments per day.  PER DAY.  Bloating my database...

Incredible Demos

  • By
    Spyjax:  Ajax For Evil Using Dojo

    The idea of Spyjax is nothing new. In pasts posts I've covered how you can spy on your user's history with both MooTools and jQuery. Today we'll cover how to check user history using the Dojo Toolkit. The HTML For the sake of this...

  • By
    CSS :target

    One interesting CSS pseudo selector is :target.  The target pseudo selector provides styling capabilities for an element whose ID matches the window location's hash.  Let's have a quick look at how the CSS target pseudo selector works! The HTML Assume there are any number of HTML elements with...

Discussion

  1. unleashit

    Nice helpful tidbit as usual. Do you know why they chose to add a brand new API over just supporting onresize on elements other than window? If you read the docs for onresize you can see that at one time some browsers supported it: https://developer.mozilla.org/en-US/docs/Web/API/Window/resize_event

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