What is JavaScript performance optimization?

Key JavaScript performance optimization techniques: Reduce DOM manipulation — batch DOM changes, use DocumentFragment, avoid layout thrashing (don't read and write DOM properties alternately). Avoid memory leaks — clean up event listeners, clear timers, use WeakMaps. Use efficient data structures — Set/Map for O(1) lookups instead of array searches. Debounce/throttle event handlers. Lazy loading — dynamic imports (import()) for code splitting, defer non-critical scripts. Web Workers — move CPU-intensive computation off the main thread. Memoization — cache expensive function results. Virtual DOM — frameworks batch updates and diff the tree. requestAnimationFrame for animations instead of setTimeout. Avoid synchronous XHR. Profile first — use Chrome DevTools Performance tab, lighthouse, and the performance API (performance.now(), performance.mark()) to identify actual bottlenecks before optimizing. Premature optimization is the root of all evil — measure, identify the bottleneck, then optimize.