frontend-bootcamp/step2-04/README.md

Step 2.4

Lessons | Exercise | Demo

Testing TypeScript code with jest. jest is a test framework made by Facebook and is very popular in the React and the wider JS ecosystem. We will work on implementing simple unit tests here in this exercise.

https://jestjs.io/

jest Features

• Multi-threaded and isolated test runner
• Provides a "fake" browser environment if needed (window, document, DOM, etc).
• Snapshots: show API or large object changes along side code changes in pull requests
• Code coverage is integrated (--coverage)
• Very clear error messages of where the test failures occur
• By default, will simulate a "good enough" browser environment called JSDOM

How to use jest

• using create-react-app or other project generators, jest should already be preconfigured. Run npm test usually will trigger it!
• needs jest.config.js
• jsdom might not have enough API from real browsers, for those cases, polyfills are required. Place these inside jest.setup.js and hook up the setup file in jest.config.js
• in order to use enzyme library to test React Components, more config bits are needed inside jest.setup.js

What does a test look like?

// describe(), it() and expect() are globally exported, so they don't need to be imported when jest runs these tests
describe('Something to be tested', () => {
  it('should describe the behavior', () => {
    expect(true).toBe(true);
  });
});

Test React Components by using enzyme

• use enzyme to mount() the component (as oppose to rendering)
• the mount() function will return a wrapper that can be inspected
• the wrapper has functionality like find(), simulating clicks, etc.

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

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

Advanced Topics

Mocking

Mocking functions is a large part of what makes jest a powerful testing library. jest actually intercepts module inclusion process in node.js allowing it to mock entire modules if needed. There are many ways to mock as you can 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.mock.calls.length).toBe(2);
});

Read more about jest mocking here: https://jestjs.io/docs/en/mock-functions.html

Async Testing

callback

it('tests callback functions', (done) => {
  someFunctionThatCallsDone(done));
})

promise

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

(recommended) async / await

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 with the command: npm test

Take a look at code inside demo/src:

1. index.ts is exports a few functions for a counter as well as a test for squaring numbers but demonstrates out jest uses mocks

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 on save to test file changes as well as source code changes under src

testing React applications

You can also test React Components with jest with the help of a partner library called enzyme. Take a look at the test below:

import { mount } from 'enzyme';

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');
  });
});

mount does a full mount of the component. You can use the enzyme wrapper to simulate clicks, etc.:

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

Exercise

Basic Testing

1. Run the tests by running npm test at the root of the bootcamp project

2. Look at the stack.ts for a sample implementation of a stack

3. Follow the instructions inside the stack.spec.ts file to complete the two tests

Enzyme Testing

1. Open up exercise/src/TestMe.spec.tsx

2. Fill in the blank for the missing test using enzyme concepts introduced from the demo

3. Run tests with npm test