frontend-bootcamp/step2-04

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/

• 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

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