What is the difference between blocking and non-blocking code in Node.js, and how do you keep the event loop responsive?

Blocking means the JavaScript thread cannot do anything else until the operation finishes. Non-blocking means the work is handed off and a callback runs when it completes. Because Node serves every request on one thread, one blocking call delays every other user.

What blocks:

• The synchronous fs methods — readFileSync, writeFileSync and friends.
• CPU-heavy work: hashing with a high cost factor, image resizing, large JSON.parse, sorting a huge array, complex regular expressions on long strings.
• child_process.execSync.

How to keep the loop free:

• Use the promise-based fs API for anything on a request path. Sync calls are acceptable only at startup.
• Move CPU-bound work to worker_threads, or to a queue consumed by a separate process.
• Break long loops into chunks that yield with setImmediate.
• Stream large payloads rather than buffering them.

Note: Say how you would detect it. Measuring event loop lag — with perf_hooks monitorEventLoopDelay or a simple timer drift check — and alerting on it is what turns this from theory into operations.