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Testing Frontend Applications: Setting up Next.js project for testing (Part 2)

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David Obodo
ยทOct 17, 2022ยท

8 min read

Testing Frontend Applications: Setting up Next.js project for testing (Part 2)
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Setting up a next.js project for testing is fairly easy. For this setup, we would be following the official guide on the Next.js documentation, however I would be explaining why we use certain things - hence this article.

For our setup, we would be making use of

  • Next.js
  • Jest
  • React Testing Library

I always choose React Testing Library becomes it is built in such a way that it prevents you from testing implementation details. Want to know why you shouldn't be testing implementation details? Check out this article by Kent C. Dodds

Initialize the project

Let's get on with it. Initialize your project with this command:

yarn create next-app --typescript

Install the necessary dependencies

If you are using Next.js 12 or any version higher, it has built in configuration for jest. Normally, It is wise to update versions of techs you use, so I would suggest you upgrade your Next app to version 12 or higher if it is currently a lower version. With that said, our next step would be to install jest and the associated packages we need for testing. Open your command line and run:

yarn add -D jest jest-environment-jsdom @testing-library/react @testing-library/jest-dom

Create configuration file

With our required packages installed, the next thing we need to do is to create a jest config file. Note that your jest configuration could exist in your package.json file as a key value pair; however, for seperation of concerns let's store it in a seperate file. Since configuration files really do not really need type checking, you can simply create a jest.config.js file. However since we already make use of typescript, lets create a jest.config.ts file instead. In order for the us to create a typescript config file we need to install ts-node so run:

yarn add -D ts-node

To reiterate:

  • jest.config.js - No need for ts-node
  • jest.config.ts - Install ts-node

If we create our jest.config file as a typescript file and don't have ts-node installed, we would get an error like what we have in the screenshot below

Screenshot 2022-10-09 at 10.16.53.png

Now, in our config file write these "required" lines of code

import nextJest from "next/jest";

const createJestConfig = nextJest({
    dir: "./", //Path to our Next App, which is basically the root of our project, if your next app wasn't in the same directory as your jest config, then this value would be different
});

const config = {
    testEnvironment: "jest-environment-jsdom"
}

export default createJestConfig(config);

So, what are we doing in this file ?

First of all we import next/jest, since it already has the built in configurations we need. Secondly we make reference to the root path of our application in the createJestConfig section. Next, we create an extra config object where we would be putting any configurations we need. After that, we add the testEnvironment to the configurations we create. Finally, we export our configurations.

Create our first test

We have successfully created the "required" configurations for our test so we can spin up a test now. Following the jest default configuration to detect test files, in your root folder create a __test__ folder. After which create a simple test file called app.test.tsx. Jest would automatically look for all files that end with a test.ts, test.tsx, test.js, test.jsx extension and consider the code in it as tests.

In your app.test file, write the following code down:

import { render } from "@testing-library/react";
import Home from "../pages/index";

describe("Application", () => {
    it("renders correctly", () => {
        render(<Home />);
    });
});

What have we done here? We have created our first test by importing our index page and rendering it on our jest-dom. We have two new keywords in our file, describe and it.

  • describe : Creates a block that groups several related tests See here
  • it: This is where you write the logic for your test. "it" is an alias of the "test" keyword

With our first test written (even though we are just rendering and not actually testing anything yet), it's time to run our test. You can run the command I am about to give directly in the terminal, but for consistency sake, I like to add it to the list of scripts in the package.json file so it can be run the exact same way as other scripts. Open your package.json file and add a test script like this

Screenshot 2022-10-08 at 20.06.26.png

We have now added:

jest --watch

Like I said earlier on, you can equally simply run that command on your terminal and you would get the needed result, but I just prefer putting it in the package.json. With all finally set, lets now run the test script we just created:

yarn run test

We should now see this in our terminal as we are good to go

Screenshot 2022-10-08 at 21.36.47.png

If we run our application by entering yarn run dev in the command line, we should have an output that looks like the image below (i.e with the exception of the red circle ๐Ÿ˜„)

Screenshot 2022-10-08 at 21.45.19.png

For our first actual test, let's check that the "Next.js" text we circled in the picture above is actually shown in the DOM.

import { render, screen } from "@testing-library/react";
import Home from "../pages/index";

describe("Application", () => {
    it("renders correctly", () => {
        render(<Home />);
        expect(screen.getByText(/Next.js!/)).toBeInTheDocument();
    });
});

We have added a test assertion saying we expect the "Next.js" test to be in the Document. Unfortunately when our tests automatically compile, an error occurs. You should see something like this in your terminal

Screenshot 2022-10-09 at 09.22.47.png

The toBeInTheDocument function we used, is part of a group called "custom matchers". In that same group we have functions like toBeDisabled, toBeEnabled e.t.c. As you can see by their nomenclature, they make it very easy for you to make assertions because of their descriptive nature. Now, for our custom matchers to work we need to import "extend-expect" from our testing library dom. Like the name implies, it is used to add more functions or extend the "expect" function that we already use in our assertions. So lets import extend-expect from our testing library

import { render, screen } from "@testing-library/react";
import Home from "../pages/index";
import "@testing-library/jest-dom/extend-expect";

describe("Application", () => {
    it("renders correctly", () => {
        render(<Home />);
        expect(screen.getByText(/Next.js!/)).toBeInTheDocument();
    });
});

When our code compiles again, you would notice that the error is gone and our test now passes. Good job ๐ŸŽ‰

Set up "common" configurations

Now, even though we have just one test file in our application, it is often common to have more than one test file and usually we would need some setup to be run before each of our tests. Some of the things we might need to run before each of our test files might include:

  • Making sure we extend-expect, like we did in the last step
  • Initializing a dotenv file for environment variables needed in our app and so much more.

To have these kind of setups run before each of our tests, we need to add them into a jest.setup.ts file and then make a reference to that file in our jest.config.ts. So lets:

Create a jest.setup.ts file in the same folder as your jest.config.ts (In our case that's the root folder)

touch jest.setup.ts

Add the common setup we need to our file. For now, that is - extending expect

import "@testing-library/jest-dom/extend-expect";

Make a reference to this file in our jest.config.ts by adding setupFilesAfterEnv: ["<rootDir>/jest.setup.ts"] into our config object. Our config file should now look like this:

const nextJest = require("next/jest");

const createJestConfig = nextJest({
    dir: "./",
});

const config = {
    testEnvironment: "jest-environment-jsdom",
    setupFilesAfterEnv: ["<rootDir>/jest.setup.ts"],
};

export default createJestConfig(config);

Remove the import "@testing-library/jest-dom/extend-expect" import from our app.test.tsx, so that it is no longer referenced in the file. app.test.tsx should now look like this:

import { render, screen } from "@testing-library/react";
import Home from "../pages/index";

describe("Application", () => {
    it("renders correctly", () => {
        render(<Home />);
        expect(screen.getByText(/Next.js!/)).toBeInTheDocument();
    });
});

Restart your tests.

Hurray ๐ŸŽ‰๐ŸŽ‰๐ŸŽ‰. Everything works perfectly and our test passes.

CONCLUSION Testing next.js apps is fun, and it is something you would do often in order to ensure that your app is stable and doesn't break down unnecessarily. The setup process can be quite repetitive so having this base setup down is good.

What we have learnt:

  1. The necessary packages to help run tests in next.js.
  2. How to make typescript configs work using ts-node.
  3. The default configurations in order to make our tests to run
  4. How to add custom matchers to our app
  5. How to make some setups, available to all of our test

In the next article, we would write tests for an application we have built.

Next => #3A: What should I test?
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