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
    Introducing MooTools Templated

    One major problem with creating UI components with the MooTools JavaScript framework is that there isn't a great way of allowing customization of template and ease of node creation. As of today, there are two ways of creating: new Element Madness The first way to create UI-driven...

  • By
    How to Create a RetroPie on Raspberry Pi &#8211; Graphical Guide

    Today we get to play amazing games on our super powered game consoles, PCs, VR headsets, and even mobile devices.  While I enjoy playing new games these days, I do long for the retro gaming systems I had when I was a kid: the original Nintendo...

Incredible Demos

  • By
    CSS Ellipsis Beginning of String

    I was incredibly happy when CSS text-overflow: ellipsis (married with fixed width and overflow: hidden was introduced to the CSS spec and browsers; the feature allowed us to stop trying to marry JavaScript width calculation with string width calculation and truncation.  CSS ellipsis was also very friendly to...

  • By
    MooTools Gone Wild: Element Flashing

    If you're like me and lay awake in bed at night, you've flipped on the TV and seen the commercials: misguided, attention-starved college girls fueled by alcohol ruining their futures by flashing lame camera-men on Spring Break. Why do they do it? Attention...

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!