Shorten a URL
Tung Nguyen
10 min read
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Overview
We have our API mapped out. Now we build our first real endpoint: POST /shorten. But for now, we are going to make it return a fake answer on purpose.
Before writing any storage or code-generation logic, we need to lock down the contract. This means defining exactly what the client sends, what the server returns, and the HTTP status code. Once this contract is under test, we can safely fill in the real logic later.
In this chapter, we build a POST /shorten route that returns a 201 Created status and a hardcoded short code. No database, no generator. Just the shape of the request and response.
What These Terms Mean
Fake it till you make it: Returning the simplest hardcoded answer that passes a test, then swapping in real logic later. Our handler will just return shortCode: "abc123" for now.
Route schema: The exact data shape Fastify expects and returns. Defining this once gives us automatic validation and free Swagger documentation.
app.inject(): A Fastify method that sends a request straight through the application in memory. It skips the network entirely, making tests fast and reliable.
201 Created: The HTTP status code indicating a new resource was successfully created, as opposed to a standard 200 OK.
Why Start with a Fake?
Most developers instinctively build the generator and the database storage first, then wire up the route at the end. We are flipping that process.
The most important question right now is whether the contract is correct. Does the endpoint take a URL, return a , and hand back the exact JSON shape we agreed on in chapter 4? We do not need a database to answer that.
