Jest

Jest is a Javascript test framework built by Facebook for for use with Node/Javascript applications.

Getting Started

Install Jest using npm i --save-dev jest. Add to your package.json:

{
  "scripts": {
    "test": "jest"
  }
}

Then create a function that we can test.

example.js

function sum(a, b) {
  return a + b;
}

module.exports = sum;

Next we create a test.

example.test.js

const sum = require('./sum');

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

Now we can run the test in the terminal with npm run test.

Mocking

Simple Functions

You can mock simple functions like fetch or axios by using the jest.fn(() => {}) syntax.

const someBlackBox = jest.fn(number => 42 + number);
forEach([0, 1], someBlackBox);

// The mock function has been called
expect(someBlackBox).toHaveBeenCalled();

// The mock function is called twice
expect(someBlackBox.mock.calls.length).toBe(2);

// The first argument of the first call to the function was 0
expect(someBlackBox.mock.calls[0][0]).toBe(0);

// The first argument of the second call to the function was 1
expect(someBlackBox.mock.calls[1][0]).toBe(1);

// The return value of the first call to the function was 42
expect(someBlackBox.mock.results[0].value).toBe(42);

You can force certain return values, as well.

const someOtherBlackBox = jest.fn();
console.log(someOtherBlackBox()); // undefined

someOtherBlackBox.mockReturnValueOnce(10).mockReturnValueOnce('x').mockReturnValue(true);
console.log(
  someOtherBlackBox(), // 10
  someOtherBlackBox(), // 'x'
  someOtherBlackBox(), // true
  someOtherBlackBox()  // true
);

expect(someOtherBlackBox.mock.calls.length).toBe(4);
// etc. etc.

Projects/Multiple Environments[4-5]

If you have elements in your application that need distinct or special test environments, use projects. This will allow you to use whatever configuration you want on whatever tests you want. Make sure you set up the correct testMatch values, or you will have a bad time.

References

1. https://jestjs.io/
2. https://jestjs.io/docs/mock-functions
3. https://www.leighhalliday.com/mock-fetch-jest
4. https://stackoverflow.com/questions/41318115/testing-two-environments-with-jest
5. https://jestjs.io/docs/configuration#projects-arraystring--projectconfig
6. https://scribe.rip/swlh/mock-a-library-in-jest-and-typescript-a8bec189efc3

Last modified: 202401040446