Fastify Web Framework

Common errors

• EADDRINUSE: address already in use :::3000

  cause The specified port (e.g., 3000) for the Fastify server is already being used by another process.
  fix Close the application currently using the port, or configure Fastify to listen on a different available port (e.g., `fastify.listen({ port: 8080 })`).

• {"statusCode":404,"error":"Not Found","message":"Route GET /your-path not found"}

  cause A request was made to an endpoint that does not have a defined Fastify route handler.
  fix Define a `fastify.get()`, `fastify.post()`, or other HTTP method handler for the requested path, or check for typos in the route path.

• FastifyError [FST_ERR_CTP_INVALID_MEDIA_TYPE]: The Content-Type header is invalid. It must be in the form 'type/subtype[; parameter=value]'

  cause An incoming request had a `Content-Type` header that did not conform to RFC 9110, specifically since Fastify v5.7.2 implemented stricter parsing.
  fix Ensure client requests send well-formed `Content-Type` headers. If processing non-standard types is required, register a custom content type parser using `fastify.addContentTypeParser()`.

• FastifyError [FST_ERR_REP_INVALID_PAYLOAD]: The payload is not a string or a buffer.

  cause `reply.send()` was called with a payload that Fastify cannot serialize or send directly (e.g., a non-plain object, a promise that doesn't resolve to a serializable type, or an unsupported stream).
  fix Ensure the payload passed to `reply.send()` is a plain object, array, string, Buffer, or a Node.js readable stream. Fastify automatically JSON-stringifies plain objects/arrays.

Warnings

• breaking Fastify now strictly enforces `Content-Type` header parsing as per RFC 9110. Malformed or non-standard `Content-Type` headers that previously worked may now be rejected.
  Fix: Ensure all incoming `Content-Type` headers strictly adhere to RFC 9110. Consider adding custom content-type parsers if legacy or non-standard types are necessary.
• breaking Upgrading from Fastify v4 to v5 introduces significant breaking changes. The `main` branch on GitHub now exclusively tracks v5.
  Fix: Consult the official Fastify v5 migration guide on `fastify.dev` for detailed steps and a comprehensive list of breaking changes before upgrading.
• gotcha Fastify regularly releases security patches for critical vulnerabilities (CVEs). It is essential to keep your Fastify package updated to the latest stable version to ensure application security.
  Fix: Regularly update your Fastify dependency to the latest patch release (e.g., `npm update fastify` or `yarn upgrade fastify`) and monitor Fastify's security advisories.

Install

• npm install fastify npm
• yarn add fastify yarn
• pnpm add fastify pnpm

Imports

• Fastify
  wrong:

  const Fastify = require('fastify')

  correct:

  import Fastify from 'fastify'

Quickstart

This example sets up a basic Fastify server listening on port 3000 and defines a root route that returns a JSON object.

import Fastify from 'fastify';

const fastify = Fastify({
  logger: true
});

fastify.get('/', async (request, reply) => {
  reply.send({ hello: 'world' });
});

fastify.listen({ port: 3000 }, (err, address) => {
  if (err) {
    fastify.log.error(err);
    process.exit(1);
  }
  console.log(`Server listening on ${address}`);
});