The Resize Observer API is used to monitor DOM element size changes and provides more efficient and accurate feedback. Compared with old methods such as window.resize event or polling detection, it directly observes specific elements and is suitable for responsive layout, chart redrawing, lazy loading and other scenarios. The basic usage includes creating an instance and passing in a callback function, specifying the target element through the observe() method, and supporting multiple element observation and cleaning operations. Note when using it: it is not triggered immediately during initialization, the browser may throttle the call frequency, avoid heavy tasks in callbacks, and some old environments need to introduce polyfills, and ensure that the actual usage scenarios are tested.
The Resize Observer API is a browser feature that lets you watch for changes in the size of one or more DOM elements. Unlike older methods, like polling with offsetWidth or relying on window resize events, Resize Observer gives you direct, efficient feedback whenever an element's dimensions actually change — even if those changes happen dynamically or without your code explicitly triggering They.
Why You'd Want to Use Resize Observer
There are plenty of situations where knowing an element's size matters. For example:
- Responsive components that need to adjust layout based on available space
- Charts or visualizations that should redraw when their container resizes
- Lazy-loading content only when it becomes visible or reach a certain size
Before Resize Observer, developers often used workarounds like listening to window.resize , which isn't always accurate or performant. Resize Observer solves this by letting you observe specific elements instead of the whole window.
How to Set It Up (Basic Usage)
Using Resize Observer is straightforward. You create an instance of it and pass in a callback function that gets called whenever any observed element changes size.
Here's a simple example:
const observer = new ResizeObserver(entries => {
for (let entry of entries) {
console.log('Element size changed:', entry.contentSize);
}
});
observer.observe(document.getElementById('my-element'));A few things to note:
- The callback receives an array of
ResizeObserverEntryobjects - Each entry contains info like
contentSize,borderBoxSize, etc. - You can observe multiple elements by calling
observe()multiple times
If you're done observing, call unobserve(element) or observer.disconnect() to clean up.
Common Pitfalls and Tips
Like any tool, Resize Observer has some gotchas you should know about:
- It doesn't fire immediately on creation — it waits until the element's size actually changes
- Throttling behavior — browsers usually throttle how often the callback runs, which is good for performance
- Avoid doing heavy work directly in the callback — consider debuncing or deferring expensive logic
Also, not all environments support it perfectly. While modern browsers do, if you're targeting older browsers like IE11, you'll need a polyfill.
One thing to keep in mind: if you're resizing the element from JavaScript (like changing its width), Resize Observer might not trigger unless the actual layout changes — so be sure to test real-world usage scenarios.
When to Reach for Resize Observer
You don't always need it. If you're just responding to the window resizing, window.addEventListener('resize', ...) still works fine. But if you're building reusable UI components or need to react to dynamic layout changes — especially in frameworks like React or Vue — Resize Observer is the better choice.
It's especially handy in component libraries, dashboards, or any layout system that needs to adapt automatically without full-page reloads or forced re-renders.
Basically that's it.
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