# Testing Koa Server with Jest

Express has long been the _de facto_ standard and mainstream pick when you start a project in Node.js. But recently (well, not that recent), the team behind Express has started a new, shiny library called Koa.js with a different phillosophy in mind.

> **Why isn’t Koa just Express 4.0?** Koa is a pretty large departure from what people know about Express, the design is fundamentally much different, so the migration from Express 3.0 to this Express 4.0 would effectively mean rewriting the entire application, so we thought it would be more appropriate to create a new library. — [Koa vs. Express](https://github.com/koajs/koa/blob/master/docs/koa-vs-express.md)

When deciding a server framework for our Node.js based Scrum Bot project, we decide to use Koa to handle http requests. Some of the deciding factors were the ease of writing async code with the latest async/await feature, and also the simple API it exposes.

This post, however, is a write-up of how I configure testing and code coverage with Jest. In case you live in a rock, Jest is a delightful, zero-configuration, snapshot-enabled testing platform from Facebook.


# Configuring Jest

The first step to start testing with Jest is installing it:


```sh
npm i -D ts-jest @types/jest # only if you use Typescript
```


The only mandatory package to install is just **jest** and **supertest** (for writing http request tests). However, in this post I will be using Typescript so I’ll add **ts-jest** and @**types/jest** too**.**

Actually, that’s the only thing we need to install, but we want to setup coverage reporting too, so there are additional things to configure. Let’s add these lines into our **package.json** file:

%[https://gist.github.com/anonymous/6c0d9c9bf82b47723dc7b2049c3438cc#file-package-json-md]

Those configuration are pretty much self-explainable. Then we set **collectCoverage** to true to activate the coverage reporting and define what files counts. To notes, the only needed configuration are the **collectCoverage** and optionally **collectCoverageFrom**. Other than that, the configuration are needed for Typescript integration or just nice to have (e.g. define reporters, threshold coverage, ignore folders).

And that’s it. Let’s write some test!

# Writing your first test

Let’s assume these are the current state of the project:

%[https://gist.github.com/anonymous/89f49a2e6791cbf2583dd9f3be57357b#file-project-state-md]

Let’s start with writing the test for our ‘/’ routes. We can use **supertest** to assert that our routes returns a ‘Hello World’ response. Let’s create an **app.test.ts** file:

%[https://gist.github.com/anonymous/c0d2150aded9f127c5d497f09bd0f1f6#file-app-test-ts]

Well that’s easy. How about we sprinkle some snapshot since we already use Jest?

%[https://gist.github.com/anonymous/52ad6922b34821c88b9fdfc5282e4c69#file-app-test-ts]

With snapshot, we don’t have to manually track each difference when we edit it in the future, we can just confirm that we want to replace the snapshot with the new one. Now you can run your test with:


```sh
npx jest
```


Or better, change your **package.json** scripts “test” to “jest”, then you can just run:


```sh
npm test
```


# Mocking your way to test the server

But what about the **server.ts** file? It’s not a simple function nor testable with **supertest**. However, we can just mock the side effect. And conveniently, jest includes the mocking feature. Let’s create **server.test.ts**:

%[https://gist.github.com/anonymous/3cc961b940c1906bacaec36e192ba625#file-server-test-ts]

And that’s it. Now we have a 100% code coverage!

![1_ubQG5ZZXqPk2amk1wgWRgA.png](https://cdn.hashnode.com/res/hashnode/image/upload/v1606289700153/7mwvRlrNN.png)*Nice greens 🍀*

# Conclusion

Testing Koa with Jest is an easy and delightful experience. With snapshots and mock functions, you can virtually create any test you want to ensure a smooth delivery and integration, especially if you use CI/CD.