3. Jest

What is Jest?

• Jest is a popular testing framework for JavaScript, developed by Facebook.
  • It can be conveniently installed via npm
• It’s primarily used for testing JavaScript and React applications.
• Jest has built-in support for mocking, assertions and test running, which help make it easier to write and manage your tests
  • Mocking: A mock is a simulated version of function/module/object used during testing to simulate some real code
    • The idea is that using a mock helps you isolate your test from those outside dependencies and ensures you’re only dependent on the code you actually want to test
  • Assertion: An assertion is a statement in a test that verifies that a value matches what is expected
    • This is how your verify what you wanted to happen actually happened
• Jest is also fairly simple to use and can work fresh out of the box with most Javascript projects

Setting up Jest**

• Jest is installed as a development dependency using NPM, npm install --save-dev jest
  • We use the --save-dev flag as jest is a development dependency, we only need it to develop our package and it isn’t important to actually run the package
• To run the tests add a test command to the scripts object in your package.json:

  "scripts": {
      "test": "jest"
  }
• Now, when you run jest it will look for:
  • Any files ending in .test.js, or .spec.js
  • Any files in a folder named __tests__
• Then, in the files it finds, jest will look for it’s valid test commands and run those tests if it finds them
• Example test file: sum.test.js:

  const sum = require('./sum');
      test('adds 1 + 2 to equal 3', () => {
      expect(sum(1, 2)).toBe(3);
  });
• After the tests run, Jest will output the results so you can see how your tests did

Common Jest Features

• Assertions

  • As mentioned earlier, jest has a lot of support for assertions, most of which are accessed this way:
    • expect(someValue).toBe(expectedValue)
  • Some other common assertions include:
    • toBe(): Exact equality.
    • toEqual(): Deep equality for objects.
    • toContain(): Checks if an array or string contains a value.
    • toThrow(): Ensures a function throws an error.
    • You can find even more in the jest documentation, however the above four will likely cover most simple cases

• Mocks:

  • As mentioned, Jest can mock: functions, modules, and even entire libraries to isolate the unit under test.
  • Useful for testing code that relies on external services or APIs.
  • Example: Mocking a function:

      const myMock = jest.fn();
      myMock.mockReturnValueOnce(10).mockReturnValueOnce(20);

• Test Suites:

  • You can group related tests together using describe() blocks.
  • Example:

      describe('Math functions', () => {
        test('adds 1 + 2 to equal 3', () => {
          expect(sum(1, 2)).toBe(3);
        });
      });

• Setup and Teardown:

  • Use beforeEach(), afterEach(), beforeAll(), and afterAll() to run code before or after tests.
  • Example: Cleaning up a mock after each test:

      afterEach(() => {
        jest.clearAllMocks();
      });