Shorten a URL

The instinct is to build the generator and the storage first, then wire up the route. We're going to flip that, and it's worth knowing why.

The first question that matters is whether the contract is right: does the endpoint take a URL, return 201, and hand back the shape we agreed on in chapter 4? You don't need a database to answer that.

By faking the short code, we get the contract under test in a few lines. The test now describes exactly what a client sees — and it keeps describing that same thing as we plug in a real generator (chapter 7) and real storage (chapter 6). Neither chapter touches this test, because the contract never changes. Build storage first and you'd be testing it with no endpoint to call it through. Build the fake route first and every later chapter has a green test holding the contract steady while you swap out the internals.

Step 1: Write the Test (Red)

Start from the red branch:

This test checks the HTTP contract: send a URL, get back the shape from chapter 4. It uses app.inject() to push the request through Fastify in-process.

It's the same pattern as the health check test from chapter 3: app.inject() sends the request without opening a real port, and we assert both the status code and the full JSON body. Notice the expected shortCode is "abc123" — that fake value is the contract we're pinning down.

Run it:

Red. The test gets a 404, not a 201 — Fastify doesn't know about POST /shorten yet. That failure is what pulls us toward writing the route.

Step 2: Implement the Route (Green)

Switch to the finish branch to see the solution:

The route defines two schemas: a request body (an object with a url field) and a 201 response (so the contract is explicit and Swagger can document it).

No database, no generation logic. The short code is hardcoded to abc123, the handler echoes back the URL it received, and the short URL is built from that fake code. It's the smallest thing that makes the test pass.

Now register the route in app.ts:

Run the tests again:

Green. Both tests pass — the health check from chapter 3 and the new shorten endpoint. The contract is now stable, which is the whole point: we can swap the fake for real storage later without touching this test.

Free Swagger Docs

Because we defined schemas on the route, Swagger picks up POST /shorten automatically — just like it did for the health check in chapter 3.

Start the server:

Open http://localhost:3000/documentation. You'll see the request body, the 201 response shape, and the endpoint description, all pulled straight from the route schema.

One schema, three payoffs: validation, type safety, and documentation.

What's Next

Next we swap the hardcoded response for a real in-memory store — so the endpoint can actually remember the URLs it shortens.