The Reply Object

When you need fine control over status, headers, redirects, or content type, the reply object exposes everything you need.

4 min read Level 2/5 #fastify#reply#headers

What you'll learn

Set status with reply.code
Set headers with reply.header
Redirect, change content type, or stream a body

reply is Fastify’s wrapper around Node’s http.ServerResponse. It is chainable and async-safe.

Status & Headers

app.post('/items', async (req, reply) => {
  const item = await createItem(req.body);
  reply
    .code(201)
    .header('Location', `/items/${item.id}`)
    .header('X-Trace', req.id);
  return item;
});

reply.code sets the status. reply.header appends one header; reply.headers({ ... }) sets several at once.

Redirects

app.get('/old', async (req, reply) => {
  return reply.redirect('/new', 301);
});

redirect(url, code?) defaults to 302. The handler can return the call directly.

Content Type & Streams

import { createReadStream } from 'node:fs';

app.get('/report.xml', async (req, reply) => {
  reply.type('application/xml');
  return createReadStream('./report.xml');
});

When the return value is a Node stream, Fastify pipes it to the response without buffering.

When to Use Reply vs Return

Return a value when you just want JSON with 200 OK. Reach for reply whenever you need a non-default status, custom headers, a redirect, a stream, or a non-JSON content type.

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