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

// ❌ WRONG - execution continues after send
fastify.get('/users/:id', async (request, reply) => {
  const user = await findUser(request.params.id);

  if (!user) {
    reply.code(404).send({ error: 'User not found' });
    // Code continues executing!
  }

  // This will execute even if user is null, causing double send
  reply.send({ user });
});

// ✅ CORRECT - return prevents further execution
fastify.get('/users/:id', async (request, reply) => {
  const user = await findUser(request.params.id);

  if (!user) {
    return reply.code(404).send({ error: 'User not found' });
  }

  return reply.send({ user });
});

// ❌ WRONG - mixing async with done()
fastify.addHook('preHandler', async (request, reply, done) => {
  try {
    await authenticateUser(request);
    done(); // This causes double execution!
  } catch (error) {
    done(error);
  }
});

// ✅ CORRECT - use async without done()
fastify.addHook('preHandler', async (request, reply) => {
  try {
    await authenticateUser(request);
    // No done() needed - just return or throw
  } catch (error) {
    throw error; // Fastify handles this automatically
  }
});

// ❌ WRONG - async send without returning reply
fastify.addHook('preHandler', async (request, reply) => {
  setImmediate(() => {
    reply.send({ message: 'Async response' });
  });
  // Fastify thinks handler completed without sending
});

// ✅ CORRECT - return reply to signal async send
fastify.addHook('preHandler', async (request, reply) => {
  setImmediate(() => {
    reply.send({ message: 'Async response' });
  });
  return reply; // Tells Fastify to wait for the async send
});

// ❌ WRONG - error handler doesn't check reply status
fastify.setErrorHandler(async (error, request, reply) => {
  console.error(error);
  reply.code(500).send({ error: error.message });
  // Fails if route already sent a response
});

// ✅ CORRECT - check if reply was already sent
fastify.setErrorHandler(async (error, request, reply) => {
  console.error(error);

  // Check if reply was already sent
  if (reply.sent) {
    return; // Don't try to send again
  }

  reply.code(500).send({ error: error.message });
});

// ❌ WRONG - multiple hooks sending responses
fastify.addHook('onRequest', async (request, reply) => {
  if (!request.headers.authorization) {
    return reply.code(401).send({ error: 'Unauthorized' });
    // This sends a response
  }
});

fastify.get('/data', async (request, reply) => {
  const data = await fetchData();
  return reply.send({ data });
  // This tries to send again if auth failed
});

// ✅ CORRECT - only one sends, others throw errors
fastify.addHook('onRequest', async (request, reply) => {
  if (!request.headers.authorization) {
    throw new Error('Unauthorized'); // Let error handler send response
  }
});

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

import fastify from 'fastify';

const app = fastify({
  logger: {
    level: 'debug', // Enable debug logging
    prettyPrint: true
  }
});

// Or use environment variable
// LOG_LEVEL=debug npm start