Implementing a Testing Framework

This week I managed to add a testing framework to my open source project, TILerator. Since it's a JavaScript project, I decided to use Jest as my testing framework.

What is Jest?

Jest is a JavaScript testing framework developed by Facebook, and is widely used for testing JavaScript and React applications. I have been exposed to Jest in other projects and along with having a detailed documentation, it is quite easy to install.

Setting up Jest

Jest is needed only for testing purposes so install it as a dev dependency.

npm install --save-dev jest

Next, add a test script to your package.json file to be able to run tests using Jest.

"scripts": {
    "test": "jest --",
    "test:watch": "jest --watch --"
  }

Finally, create a testing directory to hold all of you tests.

Writing Tests

For my first test, I decided to test out a function that receives a string and checks if a certain regex pattern can be matched, and if it can, it adds an HTML <b></b> tags to the string. I wrote 5 tests to check on the function's behaviour with certain inputs.

describe('markdownToHTML tests', () => {
    test('should return a string with <b> tags if part of string is encompassed between **', () => {
        expect(markdownToHTML('**sentence 1**  sentence 2  **sentence 3**')).toBe('<b>sentence 1</b>  sentence 2  <b>sentence 3</b>');
    });

In order to test out the core functionality of the program, I also wrote 3 tests to confirm if the correct output would be returned. There was one hiccup when writing these tests, as within the core functions, I used process.exit() to exit the program on certain conditions, and if it is called within a Jest test, it will immediately stop the test runner which could lead to incomplete test results.

After reading the documentation to combat this, I found out about mock functions and spyOn(). Using mock functions and mockImplmentation(), we can define the default implementation of a mock function, and the spyOn() function can be used to monitor the behaviour of the function. Combining these functions resulted in being able to monitor process.exit() to check the exit code that was outputted while not actually exiting the process.

 test("should exit with exit code -1 and error message if invalid file path is given", async () => {
    const mockStdErr = jest.spyOn(console, "error").mockImplementation();
    const mockExit = jest.spyOn(process, "exit").mockImplementation();

The testing commit can be found here: fc128e0

Learning Outcomes

Despite working with Jest before in other JavaScript projects, there was still a lot that I learned on the capabilities of Jest. Additionally, from writing all those tests, I improved my understanding of TILerator. Testing should be a fundamental process that is used within your project to not only identify bugs, but to also create reliable software. If you have a JavaScript project that needs a testing framework, try out Jest!