How do you handle exceptions in asynchronous code in Node.js?

Handling exceptions in asynchronous code in Node.js requires a different approach than synchronous code, because try/catch only works with synchronous blocks unless you're using async/await.

Here’s how to handle exceptions properly in different async scenarios:

✅ 1. Using async/await with try/catch

This is the most modern and readable method.

async function fetchData() {
  try {
    const data = await someAsyncFunction();
    console.log(data);
  } catch (err) {
    console.error('Error occurred:', err.message);
  }
}

✅ 2. Handling Promise Rejections

If you're using .then()/.catch(), handle errors in .catch().

someAsyncFunction()
  .then(result => {
    console.log(result);
  })
  .catch(err => {
    console.error('Caught error:', err.message);
  });

✅ 3. Global Unhandled Rejection Handling

Catch unhandled promise rejections globally (useful but shouldn't replace proper handling):

process.on('unhandledRejection', (reason, promise) => {
  console.error('Unhandled Rejection:', reason);
  // Optionally shut down gracefully
});

✅ 4. Callback Pattern (Legacy-style)

In older APIs using callbacks, check for errors manually:

fs.readFile('file.txt', (err, data) => {
  if (err) {
    console.error('Read error:', err);
    return;
  }
  console.log(data.toString());
});

✅ 5. Using Middleware in Express.js

For async route handlers, use a wrapper or next(err) to pass errors:

app.get('/user', async (req, res, next) => {
  try {
    const user = await getUser();
    res.json(user);
  } catch (err) {
    next(err); // Pass to Express error handler
  }
});

Or use a helper like:

const asyncHandler = fn => (req, res, next) => {
  Promise.resolve(fn(req, res, next)).catch(next);
};