Node.js Error Handling

What is Error Handling in Node.js?

Node errors arrive in three ways: thrown exceptions in synchronous code, rejected promises in async code, and error events on streams and emitters. Each needs a different mechanism, and missing any one crashes the process.

Error Handling example

// synchronous
try {
  JSON.parse(raw);
} catch (err) {
  console.error('Invalid JSON:', err.message);
}

// async
try {
  const data = await readFile('config.json', 'utf8');
} catch (err) {
  if (err.code === 'ENOENT') console.error('Config file missing');
  else throw err;
}

// emitter — an unhandled 'error' event crashes the process
stream.on('error', err => console.error('Stream failed:', err));

Custom errors carry useful context

class AppError extends Error {
  constructor(message, statusCode = 500, code = 'INTERNAL') {
    super(message);
    this.name = 'AppError';
    this.statusCode = statusCode;
    this.code = code;
    Error.captureStackTrace(this, this.constructor);
  }
}

throw new AppError('Job not found', 404, 'JOB_NOT_FOUND');

Key points to remember

• Check err.code rather than parsing err.message — messages change between versions.
• Handle process-level unhandledRejection and uncaughtException for logging, then exit.
• Never swallow an error silently; log it with enough context to act on.

Error codes you will meet constantly

Common mistakes with Error Handling

• An unhandled promise rejection, which terminates the process in modern Node.
• try/catch around a callback-style API, which never catches anything.