Route parameters, query and body

A handler gets the request object as its first argument — everything the client sent. Most often you take three things from it: route parameters, the query string (query — the part of the address after ?) and the request body (body — data the client sent with POST or PUT).

Route parameters

The part of the address with a colon ends up in request.params. Add a single-post route to src/routes/posts.js:

// src/routes/posts.js
app.get('/posts/:id', async (request) => {
  const id = Number(request.params.id);
  return posts.find((post) => post.id === id);
});

Open http://localhost:3000/posts/2 — the second post appears. Parameters always arrive as strings, so id is converted to a number.

A route can have several parameters:

// src/routes/comments.js
export default async function commentRoutes(app) {
  app.get('/posts/:id/comments/:commentId', async (request) => {
    const { id, commentId } = request.params;
    return { id, commentId };
  });
}

Do not forget to register the new file in src/app.js with app.register(commentRoutes). The address /posts/1/comments/7 returns {"id":"1","commentId":"7"}.

The query string

Fastify collects the parameters after ? into the request.query object:

// src/routes/posts.js
app.get('/posts', async (request) => {
  const limit = Number(request.query.limit ?? 10);
  return posts.slice(0, limit);
});

The address /posts?limit=1 returns only the first post, and /posts without the parameter returns up to ten posts.

The request body

If the client sent JSON with the Content-Type: application/json header, Fastify parses it into request.body:

// src/routes/posts.js
app.post('/posts', async (request) => {
  const { title, body } = request.body;
  const post = { id: posts.length + 1, title, body };
  posts.push(post);
  return post;
});

Send a new post from the terminal:

curl -X POST http://localhost:3000/posts \
  -H "Content-Type: application/json" \
  -d '{"title":"New post","body":"Some text"}'

The server returns the post with id 3, and it shows up in the /posts list — until the server restarts.

What else request has

So far the data is not checked at all: a client can send an empty title or a string instead of a number. Schema validation is added in the chapter on schemas.

Common mistake

Comparing a parameter with a number without converting it never matches:

// src/routes/posts.js
// Wrong: '2' === 2 is false.
posts.find((post) => post.id === request.params.id);

Convert parameters with Number() — or set the type in a schema and Fastify does it for you.