Node.js Error Handling

Backend code should fail clearly.

In Node.js, errors can come from synchronous code, callbacks, promises, streams, HTTP requests, and databases.

Synchronous errors

try {
  JSON.parse('{bad json}')
} catch (error) {
  console.error('Invalid JSON', error.message)
}

Promise errors

async function loadUser(id) {
  try {
    const user = await getUserFromDb(id)
    return user
  } catch (error) {
    console.error('Failed to load user', error)
    throw error
  }
}

Callback errors

Many older Node APIs use error-first callbacks.

fs.readFile('notes.txt', 'utf8', (error, data) => {
  if (error) {
    console.error(error)
    return
  }

  console.log(data)
})

The first argument is usually the error.

Operational vs programmer errors

Operational errors are expected runtime failures:

• database unavailable,
• file missing,
• invalid user input,
• network timeout.

Programmer errors are bugs:

• undefined variable,
• wrong function call,
• invalid assumption.

Handle operational errors gracefully. Fix programmer errors in code.

Interview answer

Node.js errors can be synchronous, callback-based, promise-based, or stream-based. Synchronous errors use try/catch, promise errors use .catch or try/catch with async/await, and callback APIs often use error-first callbacks. Good backend code handles operational errors clearly and does not silently swallow failures.