Lifecycle Hooks

Hook points in Fastify

Per request, hooks run in this order: onRequest, preParsing, preValidation, preHandler, then preSerialization, onSend and onResponse. Use them for middleware-like logic.

Common hooks

Register hooks globally or per-route. They run in sequence and can modify request/response.

// Global hook: log every request
fastify.addHook('onRequest', async (request) => {
  request.startTime = Date.now();
  request.log.info(`${request.method} ${request.url}`);
});

// Global hook: add timing header
fastify.addHook('onSend', async (request, reply, payload) => {
  reply.header('X-Response-Time', `${Date.now() - request.startTime}ms`);
  return payload;
});

// Per-route hook: validate auth token
fastify.get('/admin', 
  { onRequest: async (req, reply) => {
    const token = req.headers.authorization;
    if (!token) return reply.status(401).send({ error: 'Unauthorized' });
  }},
  async () => ({ message: 'Admin access granted' })
);

Quick check

Quick check: Which hook runs after a request is validated but before the handler?

• preHandler
• onRequest
• preValidation
• onSend

An onRequest hook

Hooks run at fixed points in the lifecycle.

• onRequest runs first, before parsing the body.
• Use async and just return, or call done() in callback style, never both.
• Here we reject requests lacking a header.

const fastify = require('fastify')();

fastify.addHook('onRequest', async (request, reply) => {
  if (!request.headers['x-api-key']) {
    reply.code(401).send({ error: 'missing key' });
  }
});

fastify.get('/', async () => ({ ok: true }));

fastify.listen({ port: 3000 });

Quick check

Quick check: Which hook runs before the body is parsed?

• preHandler
• onRequest
• onSend