Synchronous vs asynchronous code in Node

Model answer.

• Synchronous code runs step by step; each line waits for the previous one to finish. A slow step blocks everything after it.
• Asynchronous code starts an operation and continues; the result arrives later via a callback, a promise or await, while the thread serves other work.

// blocking: the whole server pauses while the file is read
app.get('/report', (req, res) => res.send(fs.readFileSync('big.csv', 'utf8')));

// non-blocking: other requests are served while the file loads
app.get('/report', async (req, res, next) => {
  try { res.send(await fs.promises.readFile('big.csv', 'utf8')); }
  catch (err) { next(err); }
});

In a start-up script (loading config once before the server listens), the sync version is simple and harmless, because nothing else is waiting. Inside a request handler, it stalls every concurrent user for as long as the read takes. For very large files, prefer streams, so memory stays flat.