How to fix Error: FST_ERR_REP_ALREADY_SENT: Reply was already sent (reply.send called twice) in Node.js

// Bad - continues after reply.send()
fastify.get('/user/:id', async (request, reply) => {
  if (!request.params.id) {
    reply.status(400).send({ error: 'ID required' });
  }
  // Code continues, tries to send another response
  const user = await getUser(request.params.id);
  reply.send(user);
});

// Good - return after reply method
fastify.get('/user/:id', async (request, reply) => {
  if (!request.params.id) {
    return reply.status(400).send({ error: 'ID required' });
  }
  const user = await getUser(request.params.id);
  return reply.send(user);
});

// Bad - multiple response paths without returns
fastify.get('/data', async (request, reply) => {
  const data = await fetchData();
  if (data.status === 'error') {
    reply.status(500).send({ error: data.message });
  }
  reply.send(data);
});

// Good - if/else ensures only one path executes
fastify.get('/data', async (request, reply) => {
  const data = await fetchData();
  if (data.status === 'error') {
    return reply.status(500).send({ error: data.message });
  }
  return reply.send(data);
});

// Bad - race condition with setTimeout
fastify.get('/process', async (request, reply) => {
  setTimeout(() => {
    reply.send({ delayed: true });
  }, 100);

  const result = await processData();
  reply.send(result); // Might happen before or after timeout
});

// Good - await all async operations
fastify.get('/process', async (request, reply) => {
  const result = await processData();
  return reply.send(result);
});

// Or if you need delayed response
fastify.post('/async-task', async (request, reply) => {
  return reply.send({ taskId: 'xyz', status: 'processing' });
  // Don't continue trying to send after this
});

// Bad - both hook and handler send
fastify.addHook('onRequest', async (request, reply) => {
  if (!request.headers.authorization) {
    reply.status(401).send({ error: 'Unauthorized' });
  }
  // Falls through to handler which also sends
});

fastify.get('/protected', async (request, reply) => {
  reply.send({ data: 'secret' });
});

// Good - return from hook if sending response
fastify.addHook('onRequest', async (request, reply) => {
  if (!request.headers.authorization) {
    return reply.status(401).send({ error: 'Unauthorized' });
  }
});

fastify.get('/protected', async (request, reply) => {
  return reply.send({ data: 'secret' });
});

// Bad - catches error but handler may have already sent
fastify.get('/risky', async (request, reply) => {
  try {
    const result = await riskyOperation();
    reply.send(result);
  } catch (error) {
    reply.status(500).send({ error: error.message }); // Duplicate send
  }
});

// Good - return in try block
fastify.get('/risky', async (request, reply) => {
  try {
    const result = await riskyOperation();
    return reply.send(result);
  } catch (error) {
    return reply.status(500).send({ error: error.message });
  }
});

// Or use Fastify error handler
fastify.setErrorHandler((error, request, reply) => {
  return reply.status(500).send({ error: error.message });
});

fastify.get('/risky', async (request, reply) => {
  const result = await riskyOperation();
  return reply.send(result);
});

fastify.addHook('onSend', async (request, reply, payload) => {
  console.log(`[onSend] Sending reply for ${request.method} ${request.url}`);
  return payload;
});

fastify.get('/test', async (request, reply) => {
  console.log('[Handler] About to send response');
  return reply.send({ ok: true });
});

// If you see onSend called twice for same request, you have a double-send issue