How to build your first REST API with Fastify

To build a REST API with Fastify, create an app, add a route with app.get(path, handler), and return an object or array from an async handler. Fastify sends it as JSON with status 200.

A GET /products route

In an empty folder, run:

npm init -y
npm pkg set type=module
npm install fastify

Create src/server.ts:

import Fastify from "fastify";

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

const products = [
  { sku: "ESP-5LB", name: "House Espresso 5 lb", priceCents: 5500 },
  { sku: "MUG-BLUE", name: "Speckled blue mug", priceCents: 1800 },
];

app.get("/products", async () => {
  return products;
});

await app.listen({ port: 3000 });

Add a script to package.json:

"scripts": {
  "start": "node src/server.ts"
}

This needs Node 22.18 or newer. Start the server, then call it from a second terminal:

npm start
curl http://localhost:3000/products

Output of curl:

[{"sku":"ESP-5LB","name":"House Espresso 5 lb","priceCents":5500},{"sku":"MUG-BLUE","name":"Speckled blue mug","priceCents":1800}]

The server terminal prints Server listening at http://127.0.0.1:3000 inside a JSON log line.

What each part does

• Fastify({ logger: true }) creates the app and logs each request as one JSON line.
• app.get("/products", handler) adds a route: a method, a path, and the handler that answers it.
• The handler returns the array. You don't call JSON.stringify or set a header yourself.
• app.listen({ port: 3000 }) starts the server on localhost.
• node src/server.ts runs TypeScript directly through Node's type stripping, so you don't need a build step. If you prefer plain JavaScript, name the file server.js and it works the same.

Any path you didn't define, such as /nope, gets a 404 JSON error automatically.

Common mistake

Forgetting "type": "module". Node still runs the file, but prints a MODULE_TYPELESS_PACKAGE_JSON warning and has to re-parse it as ESM, which is slower. Run npm pkg set type=module to remove the warning.

Takeaway

A Fastify route is a path plus an async function, and whatever it returns becomes the JSON response. Wire it to an npm script and you can start your API with npm start.