# Error Handling — Fastify

Source: https://www.geekswithgeeks.com/en/fastify/ff-error-handling

> Gracefully handle errors and control error responses.

## Error handling strategy

Throw errors in route handlers; Fastify's built-in error handler catches them and returns appropriate status codes and responses.

## Custom error class

Define a custom error class to control status codes and payloads.

```javascript
class AppError extends Error {
  constructor(message, statusCode = 500) {
    super(message);
    this.statusCode = statusCode;
  }
}

fastify.get('/users/:id', async (req) => {
  const user = await findUser(req.params.id);
  if (!user) {
    throw new AppError('User not found', 404);
  }
  return user;
});

fastify.setErrorHandler((err, req, reply) => {
  const statusCode = err.statusCode || 500;
  reply.status(statusCode).send({ error: err.message });
});
```

## Async error handling

Fastify automatically catches errors thrown in async handlers—no need for try/catch wrappers.