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135 lines
5.2 KiB
Markdown
135 lines
5.2 KiB
Markdown
# Bonus: Testing TypeScript code with Jest (Demo)
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[Lessons](../../) | [Exercise](../exercise/)
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[Jest](https://jestjs.io/) is a test framework made by Facebook and is very popular in the React and wider JS ecosystems.
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In this exercise, we will work on implementing simple unit tests using Jest.
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## Jest features
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- Multi-threaded and isolated test runner
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- Provides a fake browser-like environment if needed (window, document, DOM, etc) using [jsdom](https://github.com/jsdom/jsdom)
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- 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.
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- Code coverage is integrated (`--coverage`)
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- Very clear error messages showing where a test failure occurred
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## How to use Jest
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Using `create-react-app` or other project generators, Jest should already be pre-configured. Running `npm test` usually will trigger it!
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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](https://jestjs.io/docs/en/getting-started).
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## What does a test look like?
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```ts
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// describe(), it() and expect() are globally exported,
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// so they don't need to be imported in each test file
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describe('Something to be tested', () => {
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it('should describe the behavior', () => {
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expect(true).toBe(true);
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});
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});
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```
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- `describe()` takes a string describing the thing to be tested (often a component or file name) and a function which runs tests.
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- `it()` takes a string describing the behavior to be tested and a function to run the test.
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- `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](https://jestjs.io/docs/en/expect).
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> 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."
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## Testing React components using Enzyme
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[Enzyme](https://airbnb.io/enzyme/) is made by Airbnb and provides utilities to help test React components.
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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.
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`mount()` returns a wrapper that can be inspected and provides functionality like `find()`, simulating clicks, etc.
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The following code demonstrates how Enzyme can be used to help test React components.
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```tsx
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import React from 'react';
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import { mount } from 'enzyme';
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import { TestMe } from './TestMe';
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describe('TestMe Component', () => {
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it('should have a non-clickable component when the original InnerMe is clicked', () => {
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const wrapper = mount(<TestMe name="world" />);
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wrapper.find('#innerMe').simulate('click');
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expect(wrapper.find('#innerMe').text()).toBe('Clicked');
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});
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});
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describe('Foo Component Tests', () => {
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it('allows us to set props', () => {
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const wrapper = mount(<Foo bar="baz" />);
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expect(wrapper.props().bar).toBe('baz');
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wrapper.setProps({ bar: 'foo' });
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expect(wrapper.props().bar).toBe('foo');
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wrapper.find('button').simulate('click');
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});
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});
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```
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## Advanced topics
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### Mocking
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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.
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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.
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To mock a function:
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```ts
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it('some test function', () => {
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const mockCallback = jest.fn(x => 42 + x);
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mockCallback(1);
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mockCallback(2);
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expect(mockCallback).toHaveBeenCalledTimes(2);
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});
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```
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Read more about jest mocking [here](https://jestjs.io/docs/en/mock-functions.html).
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### Async testing
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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.
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```ts
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// Callback
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it('tests callback functions', (done) => {
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setTimeout(() => {
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done();
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}, 1000);
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});
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// Returning a promise
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it('tests promise functions', () => {
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return someFunctionThatReturnsPromise());
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});
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// Async/await (recommended)
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it('tests async functions', async () => {
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expect(await someFunction()).toBe(5);
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});
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```
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# Demo
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## Jest basics
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In this repo, we can start an inner loop development of tests by running `npm test` from the root of the `frontend-bootcamp` folder.
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Take a look at code inside `demo/src`:
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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.
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2. `multiply.ts` is a contrived example of a function that is exported
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3. `index.spec.ts` is the test file
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Note how tests are re-run when either test files or source files under `src` are saved.
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