What does it mean to block the event loop? Give examples and explain how to avoid it.

Blocking the event loop means one callback runs so long that nothing else can run. The event loop runs only one callback at a time. While a slow callback runs, Node handles no other request, timer or finished I/O until it returns. So every client waits longer.

Common causes:

• Synchronous APIs in a request handler, like fs.readFileSync or crypto.pbkdf2Sync.
• JSON.parse or JSON.stringify on very large payloads.
• Regular expressions with catastrophic backtracking: the regex tries a huge number of paths on some inputs. Attackers can use this, which is called ReDoS (Regular expression Denial of Service).
• Heavy loops, such as sorting huge arrays or processing images.

How to avoid it:

• Use the async versions of APIs.
• Limit input sizes.
• Use safe regexes.
• Move CPU-heavy work to worker_threads or a separate service.
• For medium-sized work, split it into chunks and call setImmediate between them, so other callbacks get a turn.

Measure the delay with perf_hooks.monitorEventLoopDelay.