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!