Testing Next.js apps with Jest

As a developer, you know how important tests are for any production-level project. Writing tests takes some time, but they will help you in the long run to solve problems in the codebase. You can also integrate these tests into GitHub Actions, so that whenever you deploy to production or someone makes a pull request, tests will run automatically and you’ll be notified of any failed tests.

Jest is an amazing, inbuilt tool to test React apps. And since Next.js released v12, it has Jest configuration inbuilt as well, powered by the Rust compiler (a highlight of the Next v12 update).

In this tutorial, we will learn how to set up Jest with Next by writing a simple test for a calculator app. We will also see what happens when we simulate a test failure, so you can decide whether or not Jest is efficient enough for your own project.

Tutorial prerequisites
Creating a new Next.js app
Building the calculator
Setting up Jest
Testing the Next.js calculator app with Jest
Testing calculator operations and simulating an error
Conclusion

Prerequisites

Working knowledge of React and Next.js
Working knowledge of how to test applications
A code editor — I prefer Visual Studio Code
Node.js installed on your machine

If you get stuck somewhere in the tutorial, feel free to refer to the GitHub repository.

Creating a new Next.js app

Navigate to a safe directory and enter the following command in the terminal to set up your new Next application:

npx create-next-app jest-tutorial

You can replace jest-tutorial with any other name for the application. Once the installation is complete, open the project in your code editor and run the following command in the terminal to fire up the development server:

npm run dev

Creating a calculator

Because we are focusing specifically on testing here, we won’t be covering on how our calculator works. However, for us to test, here is the code for the calculator.

The contents of index.js under the pages directory is as follows:

import Head from "next/head";
import Image from "next/image";
import { useState } from "react";
import styles from "../styles/Home.module.css";
export default function Home() {
  const [num1, setNum1] = useState(0);
  const [num2, setNum2] = useState(0);
  const [result, setResult] = useState(0);
  const add = () => {
    setResult(parseInt(num1) + parseInt(num2));
  };
  const subtract = () => {
    setResult(parseInt(num1) - parseInt(num2));
  };
  const multiply = () => {
    setResult(parseInt(num1) * parseInt(num2));
  };
  const divide = () => {
    setResult(parseInt(num1) / parseInt(num2));
  };
  return (
    <div className={styles.container}>
      <Head>
        <title>Create Next App</title>
        <meta name="description" content="Generated by create next app" />
        <link rel="icon" href="/favicon.ico" />
      </Head>
      <div className={styles.result} data-testid="result">
        {result}
      </div>
      <input
        type="number"
        className={styles.input}
        data-testid="num1"
        value={num1}
        onChange={(e) => setNum1(e.target.value)}
      />
      <input
        type="number"
        className={styles.input}
        data-testid="num2"
        value={num2}
        onChange={(e) => setNum2(e.target.value)}
      />
      <button onClick={add} className={styles.button} data-testid="add">
        Add
      </button>
      <button
        onClick={subtract}
        className={styles.button}
        data-testid="subtract"
      >
        Subtract
      </button>
      <button
        onClick={multiply}
        className={styles.button}
        data-testid="multiply"
      >
        Multiply
      </button>
      <button onClick={divide} className={styles.button} data-testid="divide">
        Divide
      </button>
    </div>
  );
}

One thing to note about in the above code is the data-testid attribute added to elements, such as a result area, input fields, and buttons. This ID helps us identify the specific field in our test files and perform actions on them while testing. These IDs won’t make any difference in production.

Here are the contents for Home.module.css under the styles directory:

.container {
  padding: 0 2rem;
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  height: 100vh;
  background-color: #121212;
  color: white;
}
.result {
  font-size: 4rem;
  font-weight: bold;
  margin-bottom: 1rem;
}
.input {
  margin: 0.5rem 0;
  padding: 0.5rem;
  font-size: large;
  width: 13rem;
  background-color: #121212;
  border: 1px solid #525252;
  color: white;
  border-radius: 10px;
}
.button {
  font-size: large;
  padding: 0.5rem;
  width: 13rem;
  margin: 0.5rem 0;
  border: 1px solid black;
  background-color: black;
  border-radius: 10px;
  color: white;
}

Now when you visit http://localhost:3000, you should see our calculator up and running:

Now, let’s move on to testing this calculator using Jest!

Setting up Jest

To set up Jest, we need to install a few required packages. Run the following command in the terminal to install them:

npm install --save-dev jest @testing-library/react @testing-library/jest-dom

Remember, we are using --save-dev here, which means these packages are included in our devDependencies and won’t be used in production.

Now, let’s create a new file to store our Jest configuration. In the project root folder, create a new file named jest.config.js with the following configuration (these configurations are from the official Next documentation):

const nextJest = require("next/jest");
const createJestConfig = nextJest({
  dir: "./",
});
const customJestConfig = {
  moduleDirectories: ["node_modules", "<rootDir>/"],
  testEnvironment: "jest-environment-jsdom",
};
module.exports = createJestConfig(customJestConfig);

Now, go to package.json and add a script called test, which runs the command jest --watch.

Your scripts should look like this:

"scripts": {
  "dev": "next dev",
  "build": "next build",
  "start": "next start",
  "lint": "next lint",
  "test": "jest --watch"
},

Now that our configurations are ready, we can move forward with writing tests.

Testing the Next.js calculator app with Jest

In the project root directory, create a new folder called tests, which will be used by Jest to look up tests. Then, create a new file called index.test.js.

Firstly, let’s import some dependencies:

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

We are importing the Home component directly from our Next app because we will be rendering it during the test.

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Now, let’s write a test to see if all elements are rendering correctly:

describe("Calculator", () => {
  it("renders a calculator", () => {
    render(<Home />);
    // check if all components are rendered
    expect(screen.getByTestId("result")).toBeInTheDocument();
    expect(screen.getByTestId("num1")).toBeInTheDocument();
    expect(screen.getByTestId("num2")).toBeInTheDocument();
    expect(screen.getByTestId("add")).toBeInTheDocument();
    expect(screen.getByTestId("subtract")).toBeInTheDocument();
    expect(screen.getByTestId("multiply")).toBeInTheDocument();
    expect(screen.getByTestId("divide")).toBeInTheDocument();
  });
});

These tests are generally human-readable. Let’s see what’s happening in the above code:

First, the describe function describes what the module is. You can give it any name you want; the name will be displayed in the console when the tests are run.

Next, the it function specifies an individual test. You can have multiple its within describe.

We are then rendering out the <Home/> component, which we imported from index.js under the pages directory. This will simulate a DOM and render the component on it.

The following expect function checks whether a condition is true or false. Its outcome pretty much tells you the result of the test. If any one of these expect statements are false, the test will fail and you will see an error message in the console.

Finally, we are using toBeInTheDocument() to check if the element actually exists in the DOM created by Jest. We provided test IDs in the page, so it’s easier for Jest to identify these elements.

Now, you can run the test. Use the following command in your terminal:

npm run test

You should see a test result like this:

 PASS  __tests__/index.test.js
  Calculator
    ✓ renders a calculator (15 ms)

Test Suites: 1 passed, 1 total
Tests:       1 passed, 1 total
Snapshots:   0 total
Time:        0.554 s, estimated 1 s
Ran all test suites.

Watch Usage: Press w to show more.

Once you run the test command, it automatically reruns the test once any file in the project is saved, so we don’t need to kill and restart the test terminal.

Testing calculator operations and simulating an error

Now, let’s create a test for checking if the numbers add properly:

it("adds numbers", () => {
  render(<Home />);
  // check if adds properly
  const num1input = screen.getByTestId("num1");
  const num2input = screen.getByTestId("num2");
  const addButton = screen.getByTestId("add");
  const resultArea = screen.getByTestId("result");
  fireEvent.change(num1input, { target: { value: 5 } });
  fireEvent.change(num2input, { target: { value: 8 } });
  addButton.click();
  expect(resultArea).toHaveTextContent("13");
});

In the above code, we are simulating typing and pressing a button in the calculator. We are also providing two numbers to add, and checking if the result the calculator provides is the same.

Once you save the file, check the terminal and your test should automatically rerun and provide the following output:

 PASS  __tests__/index.test.js
  Calculator
    ✓ renders a calculator (18 ms)
    ✓ adds numbers (9 ms)

Test Suites: 1 passed, 1 total
Tests:       2 passed, 2 total
Snapshots:   0 total
Time:        0.526 s, estimated 1 s
Ran all test suites.

Watch Usage: Press w to show more.

Perfect! Now, let’s try to simulate an error in the application. First, we will make a change in the actual calculator application so that it does not work as expected on purpose. To do so, change the add function in index.js as follows:

const add = () => {
  setResult(parseInt(num1) - parseInt(num2));
};

Here we changed the function so that it subtracts instead of adds. When you save the file, you should immediately see an error:

 FAIL  __tests__/index.test.js
  Calculator
    ✓ renders a calculator (14 ms)
    ✕ adds numbers (13 ms)

  ● Calculator › adds numbers

    expect(element).toHaveTextContent()

    Expected element to have text content:
      13
    Received:
      -3

      28 |     addButton.click();
      29 |
    > 30 |     expect(resultArea).toHaveTextContent("13");
         |                        ^
      31 |   });
      32 | });
      33 |

      at Object.<anonymous> (__tests__/index.test.js:30:24)

Test Suites: 1 failed, 1 total
Tests:       1 failed, 1 passed, 2 total
Snapshots:   0 total
Time:        0.667 s, estimated 1 s
Ran all test suites.

Watch Usage: Press w to show more.

Jest tried to add numbers and didn’t see a number we expected, which means there is some problem in the add function. That’s how you can check problems in your code when a test fails. Revert the changes you made in index.js file so that the contents are back to the original working order.

Now, let’s add a few more tests to check if all the operations are working well. Here’s the entire code for index.test.js:

import Home from "../pages/index";
import "@testing-library/jest-dom";
import { fireEvent, render, screen } from "@testing-library/react";
describe("Calculator", () => {
  it("renders a calculator", () => {
    render(<Home />);
    // check if all components are rendered
    expect(screen.getByTestId("result")).toBeInTheDocument();
    expect(screen.getByTestId("num1")).toBeInTheDocument();
    expect(screen.getByTestId("num2")).toBeInTheDocument();
    expect(screen.getByTestId("add")).toBeInTheDocument();
    expect(screen.getByTestId("subtract")).toBeInTheDocument();
    expect(screen.getByTestId("multiply")).toBeInTheDocument();
    expect(screen.getByTestId("divide")).toBeInTheDocument();
  });
  it("adds numbers", () => {
    render(<Home />);
    // check if adds properly
    const num1input = screen.getByTestId("num1");
    const num2input = screen.getByTestId("num2");
    const addButton = screen.getByTestId("add");
    const resultArea = screen.getByTestId("result");
    fireEvent.change(num1input, { target: { value: 5 } });
    fireEvent.change(num2input, { target: { value: 8 } });
    addButton.click();
    expect(resultArea).toHaveTextContent("13");
  });
  it("subtracts numbers", () => {
    render(<Home />);
    // check if adds properly
    const num1input = screen.getByTestId("num1");
    const num2input = screen.getByTestId("num2");
    const subtractButton = screen.getByTestId("subtract");
    const resultArea = screen.getByTestId("result");
    fireEvent.change(num1input, { target: { value: 8 } });
    fireEvent.change(num2input, { target: { value: 5 } });
    subtractButton.click();
    expect(resultArea).toHaveTextContent("3");
  });
  it("multiplies numbers", () => {
    render(<Home />);
    // check if adds properly
    const num1input = screen.getByTestId("num1");
    const num2input = screen.getByTestId("num2");
    const multiplyButton = screen.getByTestId("multiply");
    const resultArea = screen.getByTestId("result");
    fireEvent.change(num1input, { target: { value: 5 } });
    fireEvent.change(num2input, { target: { value: 8 } });
    multiplyButton.click();
    expect(resultArea).toHaveTextContent("40");
  });
  it("divides numbers", () => {
    render(<Home />);
    // check if adds properly
    const num1input = screen.getByTestId("num1");
    const num2input = screen.getByTestId("num2");
    const divideButton = screen.getByTestId("divide");
    const resultArea = screen.getByTestId("result");
    fireEvent.change(num1input, { target: { value: 20 } });
    fireEvent.change(num2input, { target: { value: 2 } });
    divideButton.click();
    expect(resultArea).toHaveTextContent("10");
  });
});

Once you save the file, you should see all the tests passing like so:

 PASS  __tests__/index.test.js
  Calculator
    ✓ renders a calculator (14 ms)
    ✓ adds numbers (6 ms)
    ✓ subtracts numbers (4 ms)
    ✓ multiplies numbers (4 ms)
    ✓ divides numbers (4 ms)

Test Suites: 1 passed, 1 total
Tests:       5 passed, 5 total
Snapshots:   0 total
Time:        0.674 s, estimated 1 s
Ran all test suites.

Watch Usage: Press w to show more.

Conclusion

After reading this article, I hope you learned how important testing is in production-level projects. Especially with huge projects, it would take a lot of time to manually test them. So, writing automated tests with Jest is a great solution. Every time you ship a feature and something breaks, you will be notified when a test fails so that you can work on a fix immediately.

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7 Replies to “Testing Next.js apps with Jest”

Somnath says:

April 13, 2022 at 10:24 am

Below error occured,

Cannot find module ‘react-dom/client’ from ‘node_modules/@testing-library/react/dist/pure.js’
1. Atharva Deosthale (@athudeosthale) says:
  
  April 14, 2022 at 10:26 am
  
  Hey Somnath! This error might have been caused because of your packages not being properly installed. Please delete the “node_modules” folder and run “npm install” again. Make sure you’re importing functions and components from the correct package.
Lin says:

May 12, 2022 at 12:28 pm

hey, the second test is failed:
` it(“adds numbers”, () => {
render()
// check if adds properly
const num1input = screen.getByTestId(“num1”)
const num2input = screen.getByTestId(“num2”)
const addButton = screen.getByTestId(“add”)
const resultArea = screen.getByTestId(“result”)
fireEvent.change(num1input, { target: { value: 5 } })
fireEvent.change(num2input, { target: { value: 8 } })
addButton.click()
expect(resultArea).toHaveTextContent(“13”)
})`

Expected element to have text content:
13
Received:
0
1. Dimasaurio says:
  
  June 9, 2022 at 2:20 pm
  
  Hello, you need change
  addButton.click()
  to
  fireEvent.click(addButton)
Lin says:

May 12, 2022 at 10:02 pm

Hi, the second test is failed, all I did was copy and paste.
Expected 13, got 0.
Gajen says:

October 28, 2022 at 10:48 pm

This if for a Next.js Project. It said “As of Jest 28 “jest-environment-jsdom” is no longer shipped by default, make sure to install it separately.” So I did “npm install –save-dev jest-environment-jsdom” and I’ve removed the line “testEnvironment: “jest-environment-jsdom”,” from the “jest.config.js” file. Then it works..!
gajendramehra says:

April 3, 2023 at 7:07 am

or you can also do await addButton.click();