ResizeObserver is a modern JavaScript API that reports changes to the dimensions of a DOM element's content or border box. Unlike the older window.resize event (which only fires when the entire viewport resizes), ResizeObserver can track size changes on any individual element—even those caused by CSS changes, dynamic content loading, or JavaScript manipulation. It's widely supported in all modern browsers (Chrome 64+, Firefox 69+, Safari 13.1+, Edge 79+).

window.resize only fires when the entire browser viewport changes size. In contrast, ResizeObserver watches individual DOM elements and detects size changes caused by any reason: CSS media queries, flex/grid layout recalculations, content insertion/removal, JavaScript DOM manipulation, or even CSS animations. This granularity makes it ideal for responsive components and dynamic layouts.

MutationObserver watches for changes to the DOM tree (child node additions/removals, attribute changes, text content updates). It does not directly report size changes. ResizeObserver specifically watches for element dimension changes. They complement each other: use MutationObserver to detect DOM mutations, and ResizeObserver to react when an element's rendered size actually changes—even if the DOM structure remains the same (e.g., CSS flex reflow).

• Responsive components: Adjust internal layout when a parent container resizes.
• Canvas/Chart resizing: Redraw charts when their container dimensions change.
• Lazy-loading placeholders: Detect when an element gains dimensions after content loads.
• Custom scrollbars: Recalculate scrollbar geometry when content area resizes.
• iframe auto-height: Adjust iframe height based on its content size.
• CSS Container Queries polyfill: Implement responsive logic based on container size.

Yes, ResizeObserver is designed to be highly performant. Unlike older polling-based approaches (using setInterval to check dimensions), ResizeObserver uses a callback-based model that fires only when actual size changes occur. The callbacks are batched and delivered in the same microtask, minimizing layout thrashing. For most use cases, the performance impact is negligible. However, avoid performing heavy synchronous work inside the callback—use requestAnimationFrame if you need to defer expensive operations.

Use observer.unobserve(element) to stop watching a specific element, or observer.disconnect() to stop observing all elements and release the observer entirely. It's good practice to call disconnect() when the observed element is removed from the DOM or when your component unmounts, to prevent memory leaks.