Jest

What is it

  • It is a Javascript test framework.
  • Can be used both in front-end and back-end, but its commonly used on the backend.
  • Can be used with: React, Node, React Native, and many others.
  • Developed by Facebook.

How to use

  • Install Jest globally in your computer using npm install jest --global.
  • Go to your project root and run npm install --save-dev jest.
  • Create a folder in the root named as __tests__.
  • Then, initialize the Jest configuration file with jest --init.

    At the root will appers a new file jest.config.js. ( Access this link to understand about each configuration )

Writing tests

  • At this point, the environment is already set, so now we can write the tests.

  • Now we need to create a file inside __tests__ folder, in my case example.test.js.

    Obs: Isnt required to end the filename with.test.js(can be just.js`), but it is interesting to keep the code well organized.

    // 'it' can be traded for 'test'
it("two plus two is four", () => {
  expect(2 + 2).toBe(4);
});

  • As you can see, with this simple way we can write trivial tests and complex one, for example, asynchronous code:

    // 'it' can be traded for 'test'
it("two plus two is four", async () => {
  try {
    const data = await fetchSomeData();
    //...//
  } catch (e) {
    expect(e).toMatch("error");
  }
});

  • In some cases, one test file can have multiple features to be tested, then we can use the describe() to 'split' the tests into categories:

    # __tests__/session.test.js
  describe('Authorization', () => {
    it('should authenticate with valid credentials', () => {
      //Some logic
    });

    it('should not authenticate with invalid credentials', () => {
      //Some logic
    });
  });

  describe('Registration', () => {
    it('should add an user in database', () => {
      //Some logic
    });

    it('should not add an user in database ', () => {
      //Some logic
    });
  });

Running tests

  • To run the test simply, you can just add a script at package.json named as test.

    ...
  "scripts": {
    "test": "jest",
  },
...

  • Then, in your terminal run npm run test. And finally tests is working!

Learning more

Useful content

  • As always the oficial docs are a great place to get more info.
  • Video About NodeJs + Jest + TDD. (Turn on the english subtitle)

Validating knowledge

  • It is always very important to put into practice what you learn, to solidify and validate de knowledge. So we strongly recommend you tom start a project and put the hands on.
  • Think about something simple, like login + authentication, o some like this.
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