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Jest

Jest is one of most popular testing frameworks for its ease of use.

Simplest way of understanding tests is to think of tests as of bits of code that check if a given function produces the expected result. Such as:

  1. import the function to test
  2. give an input to the function
  3. define what to expect as the output
  4. check if the function produces the expected output

Really, that’s it. Testing won’t be scary anymore if you think in these terms: input - expected output - assert the result.

The best way to run tests in VSCode is with the extension.

Write jest --coverage into package.json next to test. The —coverage flag makes Jest generate coverage data for tests automatically. This is useful to see which parts of your codebase lack testing. It writes this data into a coverage folder.

can also use --watchall to watch test files as they change and --verbose to see more details in the terminal.

expect("values") ... matcher

There are lots of matchers, like toBe(), toEqual(), toContain(), etc.

.toBe()

  • Checks primitive values like numbers and booleans.

.toEqual()

  • Checks values of objects and arrays

.toBeFalsy()

  • Checks if value is falsy, ie false, 0, ”, undefined etc.

.toBeTruthy()

  • Checks if value is truthy, ie not false, 0, ”, undefined etc.

.toThrow()

  • Checks if an error is thrown. Ie. invalid input etc.

Testing async code

  • Callbacks
    • The done parameter is passed to the callback.
  • Promises
    • use resolves and rejects
  • Async
    • use async and await
    • Cleaner testing

Mock Functions and Spies

Mocking isolates the unit of code which is being tested. Replace slow or unavailable functions. Use jest.fn to write a mock function.

test("mock implementation of a function", () => {
  const mock = jest.fn((x) => 42 + x);
  expect(mock(1)).toBe(43);
  expect(mock).toHaveBeenCalledWith(1);
});

Use toHaveBeenCalledWith to check if a function was called with certain arguments.

Use spy to see if a function was called. Spy.mockRestore() to restore the original function.

Helper functions

These help set up and tear down tests. e.g beforeAll, beforeEach, test, afterEach, afterAll etc. They prevent code duplication within tests.

like beforeEach, can write a new function before each test.

Resources

Jest Crash Course - a good starting point for learning about Jest. Jest for Beginners - this explains really well what testing is for, and how to use Jest.

To learn more about Jest: