frontend-bootcamp/bonus-jest/demo/README.md

Bonus: Testing TypeScript code with Jest (Demo)

Lessons | Exercise

Jest is a test framework made by Facebook and is very popular in the React and wider JS ecosystems.

In this exercise, we will work on implementing simple unit tests using Jest.

Jest features

• Multi-threaded and isolated test runner
• Provides a fake browser-like environment if needed (window, document, DOM, etc) using jsdom
• Snapshots: Jest can create text-based snapshots of rendered components. These snapshots can be checked in and show API or large object changes alongside code changes in pull requests.
• Code coverage is integrated (--coverage)
• Very clear error messages showing where a test failure occurred

How to use Jest

Using create-react-app or other project generators, Jest should already be pre-configured. Running npm test usually will trigger it!

Setting up Jest in a new project is outside the scope of this course, but if you're interested in how it works, take a look at the bootcamp project's jest.config.js and jest.setup.js files or the getting started documentation.

What does a test look like?

// describe(), it() and expect() are globally exported,
// so they don't need to be imported in each test file
describe('Something to be tested', () => {
  it('should describe the behavior', () => {
    expect(true).toBe(true);
  });
});

• describe() takes a string describing the thing to be tested (often a component or file name) and a function which runs tests.
• it() takes a string describing the behavior to be tested and a function to run the test.
• expect() takes the actual value as a parameter and returns an object with various "matcher" methods to test against an expected value/condition. toBe is just one of many available matchers.

When choosing test names, think of the strings passed to describe and it as forming a sentence. For example, inside describe('MyComponent', ...) you might have a test it('renders some text', ...), which forms the sentence a sentence describing the behavior: "MyComponent renders some text."

Testing React components using Enzyme

Enzyme is made by Airbnb and provides utilities to help test React components.

In a real app using ReactDOM, the top-level component will be rendered on the page using ReactDOM.render(). Enzyme provides a lighter-weight mount() function which is usually adequate for testing purposes.

mount() returns a wrapper that can be inspected and provides functionality like find(), simulating clicks, etc.

The following code demonstrates how Enzyme can be used to help test React components.

import React from 'react';
import { mount } from 'enzyme';
import { TestMe } from './TestMe';

describe('TestMe Component', () => {
  it('should have a non-clickable component when the original InnerMe is clicked', () => {
    const wrapper = mount(<TestMe name="world" />);
    wrapper.find('#innerMe').simulate('click');
    expect(wrapper.find('#innerMe').text()).toBe('Clicked');
  });
});

describe('Foo Component Tests', () => {
  it('allows us to set props', () => {
    const wrapper = mount(<Foo bar="baz" />);
    expect(wrapper.props().bar).toBe('baz');
    wrapper.setProps({ bar: 'foo' });
    expect(wrapper.props().bar).toBe('foo');

    wrapper.find('button').simulate('click');
  });
});

Advanced topics

Mocking

Mocking functions is a large part of what makes Jest a powerful testing library. Jest actually intercepts the module loading process in Node.js, allowing it to mock entire modules if needed.

There are many ways to mock, as you'd imagine in a language as flexible as JS. We only look at the simplest case, but there's a lot of depth here.

To mock a function:

it('some test function', () => {
  const mockCallback = jest.fn(x => 42 + x);
  mockCallback(1);
  mockCallback(2);
  expect(mockCallback).toHaveBeenCalledTimes(2);
});

Read more about jest mocking here.

Async testing

For testing async scenarios, the test runner needs some way to know when the scenario is finished. Jest tests can handle async scenarios using callbacks, promises, or async/await.

// Callback
it('tests callback functions', (done) => {
  setTimeout(() => {
    done();
  }, 1000);
});

// Returning a promise
it('tests promise functions', () => {
  return someFunctionThatReturnsPromise());
});

// Async/await (recommended)
it('tests async functions', async () => {
  expect(await someFunction()).toBe(5);
});

Demo

Jest basics

In this repo, we can start an inner loop development of tests by running npm test from the root of the frontend-bootcamp folder.

Take a look at code inside demo/src:

1. index.ts exports a few functions for a counter as well as a function for squaring numbers. We'll use this last function to demonstrate how mocks work.

2. multiply.ts is a contrived example of a function that is exported

3. index.spec.ts is the test file

Note how tests are re-run when either test files or source files under src are saved.