# Testing Routes — Fastify: Building High-Performance Node.js APIs

Source: https://www.skillbyai.com/en/fastify/ff-testing

> Test Fastify routes quickly with fastify.inject and Node's built-in test runner, with no real server.

## In-process testing

Fastify can inject requests directly without starting a real server—ideal for unit and integration tests.

## Testing with inject

Use `fastify.inject()` to send requests and verify responses.

```javascript
import test from 'node:test';
import assert from 'node:assert';
import build from './app.js';

test('GET /health returns 200', async () => {
  const fastify = await build();
  
  const response = await fastify.inject({
    method: 'GET',
    url: '/health',
  });
  
  assert.equal(response.statusCode, 200);
  assert.deepEqual(JSON.parse(response.body), { status: 'ok' });
  
  await fastify.close();
});
```

## Testing with inject, no network

`inject` simulates a request in memory using light-my-request.

- No port is opened.
- `res.statusCode` and `res.json()` read the answer.

```js
const test = require('node:test');
const assert = require('node:assert');
const Fastify = require('fastify');

test('GET /ping', async () => {
  const app = Fastify();
  app.get('/ping', async () => ({ pong: true }));
  const res = await app.inject({ method: 'GET', url: '/ping' });
  assert.strictEqual(res.statusCode, 200);
  assert.deepStrictEqual(res.json(), { pong: true });
});
```

Quick check

**Quiz:** Why prefer `app.inject()` over starting a server and calling fetch in unit tests?

- [x] It is faster and needs no free port
- [ ] It skips all hooks
- [ ] It works only for POST

*Answer:* It is faster and needs no free port. inject runs the full lifecycle in-process, so hooks and validation still apply.