How to Test Next.js Applications Using Jest
The primary objective of any development process is to build production-ready applications. To achieve this, it is essential to write code that not only fulfills the functional requirements but also remains stable and reliable over time. Testing serves as a safeguard, ensuring that applications function as expected even as new updates are made during development.
While writing comprehensive tests that encompass various edge cases may take a considerable amount of time, it helps to flag and resolve issues before they reach production environments.
Testing Next.js Applications
Writing tests is an essential and often underestimated aspect of developing robust applications. It’s easy to fall into the temptation of shipping code directly to production, relying on the belief that “you authored the code, so it must work!”
However, this approach can lead to unforeseen issues and bugs in production environments. As a result, adopting a Test-driven development (TDD) approach can help maximize your confidence in your code and minimize time spentdebugging and resolving minor bugsthat could have slipped to production.
What Is Jest?
Jestis a popular testing framework that is widely used by differentJavaScript frameworks. It provides a suite of testing features such as a powerful test runner, automatic mocking, and snapshot testing.
Ideally, these features come in handy in achieving comprehensive test coverage and ensuring the reliability of your application acrossdifferent types of tests.
Create a Next.js To-Do Application
Now, let’s delve into the process of running unit tests on a Next.js application using Jest. However, before you begin, create a Next.js project and install the required dependencies.
To get started, follow these steps:
With the project set up, and the dependencies installed, you are now ready to build the Next.js application and write unit tests using Jest.
Feel free to refer to the project’s code in thisGitHub repository.
Create the To-Do Component
In the/srcproject directory, open thepages/index.jsfile, delete the existing boilerplate Next.js code, and add the following code.
First, make the following imports and define two functions that will manage user To-Dos:addTodofunction anddeleteTodofunction.
The code utilizes theuseStatehook to initialize and update the state variables:todosandnewTodo. TheaddTodofunction adds a new To-Do to the todos list when the input value is not empty, while thedeleteTodofunction removes a specific To-Do from the list based on its index.
Now, write the code for the JSX elements rendered on the browser’s DOM.
Writing Test Cases
Before you start writing test cases, it’s crucial to configure Jest according to your specific testing requirements. This involves creating and customizing thejest.config.jsfile, which serves as the foundation for your testing setup.
In the root directory, create a newjest.config.jsfile. Then, add the following code to configure Jest accordingly:
Finally, open thepackage.jsonfile and add a new script calledtestthat will execute the commandjest –watchAllto run all the test cases and watch for any changes.
After making the update, your scripts should look like this:
With the configurations in place, proceed with writing and executing tests.
Testing the Next.js To-Do App With Jest
In the project root directory, create a new folder called__tests__. Jest will look up test files in this folder. Inside this folder, create a new file calledindex.test.js.
First, make the following imports.
Write a test to see if all the elements are rendering correctly:
The test case verifies that the To-Do app and its elements render correctly. Inside the test case, theHomecomponent is rendered using therenderfunction from the testing library.
Then, assertions are made using theexpectfunction to ensure that specific elements with test IDs, such astodo-inputare present in the rendered output. If these elements are found in the DOM, the test passes; otherwise, it fails.
Now, run the following command to execute the test.
You should see a similar response if the test passes.
Testing Different Actions and Simulating Errors
Describe these test cases to verify the functionality of the Add To-Do and Delete To-Do features.
Start by writing the test case for the Add To-Do functionality.
The code snippet above simulates user interaction by typing into an input field and clicking the add button. Then, using a mock To-Do input value, It verifies if the input value is successfully added to the To-Do list.
Save the file and check the terminal. The test should automatically rerun and log out similar test results.
To simulate a test error, modify the add button test ID and rerun the test. With this update, the test should fail and log out an error message indicating the specific error encountered.
Ideally, in a larger codebase with multiple contributors making frequent changes, testing plays a crucial role in identifying potential bugs that could lead to system failures. By testing, you can easily flag inconsistencies, such as the one demonstrated above, and resolve them during development.
Finally, write the test case for the Delete To-Do functionality.
Again, it verifies if the To-Do is successfully deleted. Save the file to execute the test.
Unit Testing Using Jest
This guide has provided you with the knowledge to write and execute simple unit tests, using a To-Do component as an example. To ensure the stability and reliability of your application’s core features and reduce the chances of unexpected problems in production environments, it is important to prioritize writing tests for your key components.
Furthermore, you could enhance your testing approach by incorporating snapshot tests and end-to-end tests.
Testing is a vital but often overlooked aspect of web development. Jest can help you test your React components and produce more reliable apps.
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