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What is the difference between concurrency and parallelism, and which one does Node.js's event loop actually provide by default? What is a callback queue (or task queue) in the context of the Node.js event loop, and how does it relate to the call stack? What does 'non-blocking I/O' mean, and how does a Node.js function like fs.readFile() achieve it compared to fs.readFileSync()? How does setImmediate() differ from process.nextTick() in terms of timing and typical use cases? How does the event loop handle a large synchronous loop (like processing a huge array with a for loop), and what problem does this cause? What does it mean that Node.js's I/O is handled by libuv, and how does libuv abstract different operating systems' I/O mechanisms? What is the difference between synchronous and asynchronous versions of the same core Node.js API, using fs.readFile vs fs.readFileSync as an example, and when is the synchronous version actually appropriate? How would you measure how 'busy' the event loop is in a running Node.js process using a simple technique? What are the distinct phases of the Node.js event loop, and what kind of callbacks does each phase execute? What is 'event loop lag' (or 'event loop delay'), and how would you monitor it in a production Node.js application?
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What is the event loop, and why does it allow Node.js to handle many concurrent operations despite being single-threaded?

Beginner

Answer

The event loop is the core mechanism that lets Node.js perform non-blocking I/O -- it continuously checks for completed asynchronous operations (a finished file read, an incoming network request) and executes their associated callbacks, allowing a single JavaScript thread to juggle many in-flight I/O operations by never blocking while waiting for any one of them, instead delegating I/O work to the OS (or libuv's thread pool) and being notified when it's done.
console.log('1: Starting');
fs.readFile('file.txt', () => console.log('3: File read complete'));
console.log('2: This runs before the file read finishes');
// Output order: 1, 2, 3 -- the file read happens in the background
// while the main thread continues executing other code
Real-world example A web server handling thousands of concurrent connections doesn't need a thread per connection like some traditional server architectures, since the event loop lets a single thread initiate many I/O operations (database queries, file reads) and handle whichever one completes first, without any of them blocking the others.

Common follow-ups: What specifically happens to a request if a piece of synchronous, CPU-heavy code runs during the event loop's processing?;How does the event loop relate to and differ from multi-threading in a language like Java?

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All 15 Event Loop & Non-blocking IO questions