How does Node.js achieve high concurrency for I/O-bound workloads despite running JavaScript on a single main thread?

Node.js's single-threaded JavaScript execution model is combined with an event-driven, non-blocking I/O architecture -- when an I/O operation (a network request, a file read, a database query) is initiated, Node.js delegates the actual waiting to the underlying operating system (or libuv's thread pool for operations lacking native OS-level async support), immediately returning control to the event loop to handle other work, rather than blocking the single thread waiting for that I/O to complete; when the I/O eventually finishes, its callback is queued to run on the main thread. This lets a single Node.js process efficiently juggle thousands of concurrent connections, since it spends almost no time idly blocked waiting on any single one.

// This doesn't block the single thread -- Node.js moves on immediately
// to handle other requests while this database query is in flight
app.get('/users/:id', async (req, res) => {
  const user = await db.users.findById(req.params.id); // non-blocking wait
  res.json(user);
});
// Meanwhile, thousands of OTHER concurrent requests can be handled
// on this same single thread while this one awaits its database response