Node.js Callbacks and Callback Hell

What is Callbacks and Callback Hell in Node.js?

A callback is a function passed to an asynchronous operation and invoked when it finishes. Node’s convention is error-first: the callback receives an error as its first argument and the result as the second.

Callbacks and Callback Hell example

fs.readFile('a.json', 'utf8', (err, a) => {
  if (err) return console.error(err);
  fs.readFile('b.json', 'utf8', (err, b) => {
    if (err) return console.error(err);
    fs.writeFile('c.json', a + b, err => {
      if (err) return console.error(err);
      console.log('done');   // three levels deep already
    });
  });
});

The same logic with async/await

try {
  const [a, b] = await Promise.all([
    readFile('a.json', 'utf8'),
    readFile('b.json', 'utf8'),
  ]);
  await writeFile('c.json', a + b);
  console.log('done');
} catch (err) {
  console.error(err);
}

Flat, readable, one error handler — and the two reads now run concurrently instead of one after the other.

Key points to remember

• Always check the error argument first and return early.
• util.promisify converts a callback API into a promise-based one.
• try/catch cannot catch an error passed to a callback.

Common mistakes with Callbacks and Callback Hell

• Forgetting return after handling an error, so the success path runs too.
• Wrapping callback code in try/catch and believing it is handled.