TypeScript Setup

Fastify ships first-class TypeScript types; route generics give you typed body, params, query, and reply with zero extra build steps.

4 min read Level 2/5 #fastify#typescript#generics

What you'll learn

Install Node and Fastify type packages
Use route generics for typed inputs
Pair with TypeBox or zod for runtime types

Fastify is written in TypeScript and exposes rich types. The default DX is good — adding a type provider makes it great.

Project Setup

npm i fastify
npm i -D typescript tsx @types/node
npx tsc --init --module nodenext --target es2022 --strict

Fastify 5 is ESM-first; set "type": "module" in package.json and use the nodenext module resolution in tsconfig.json.

Route Generics

type SearchQuery = { q: string; page?: number };
type SearchReply = { results: Array<{ id: number; title: string }> };

app.get<{ Querystring: SearchQuery; Reply: SearchReply }>(
  '/search',
  async (req) => {
    const q = req.query.q.trim();
    const page = req.query.page ?? 1;
    return { results: await search(q, page) };
  },
);

req.query, req.body, req.params, and the return type are all checked at compile time.

Type Providers

Hand-written generics drift from runtime schemas. A type provider derives types from your schema:

import { Type } from '@sinclair/typebox';
import { TypeBoxTypeProvider } from '@fastify/type-provider-typebox';

const typed = app.withTypeProvider<TypeBoxTypeProvider>();

typed.post('/users', {
  schema: {
    body: Type.Object({ email: Type.String({ format: 'email' }) }),
  },
  handler: async (req) => req.body.email.toLowerCase(),
});

One schema definition powers Ajv validation, OpenAPI docs, and compile-time types — covered in depth in the TypeBox lesson.

Schemas — Validate & Serialize →