AbortController for cancellable async operations

Need to cancel in-flight fetch requests, timeouts, or async operations when component unmounts or a newer request supersedes an older one.

AbortController provides standard cancellation: const controller = new AbortController(); fetch(url, { signal: controller.signal }); controller.abort(). Works with: fetch, addEventListener (signal option), Node.js streams. For timeouts: AbortSignal.timeout(5000). Combine: AbortSignal.any([signal1, signal2]). In React: create in useEffect, abort in cleanup.

Without cancellation, abandoned requests waste resources and cause bugs when stale responses arrive after the consumer is gone. AbortController is the web standard for cooperative cancellation.