Jest: Frontend Testing leicht gemacht @EnterJS2018

Holger Grosse-Plankermann
Jest: Frontend Testing done right

Who am I?
Developer/Consultant/Whatever
Taming the Web since the 2000
Compiled Mozilla for bonus features
Backend vs. Frontend dev
Podcaster http://autoweird.fm
Holger Grosse-Plankermann
@holgergp
http://github.com/holgergp

Show of hands
Do you test your frontend code?
•Unit Testing
•Selenium/Test Cafe etc.
•Integrated Tools (e.g. Katalon)
•Macros
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Show of hands
What test framework are you using?
•Jest
•Mocha
•Karma
5
•Jasmine
•Qunit
•Something else?

AGENDA
What is Jest
Why another library?
(Unit-) Tests in JavaScript
Where do I come from?
Where are we heading?
Top 3 reasons for Jest
Why I think Jest is awesome!

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Where do I come from
(Unit-) Testing JS

Test Pyramid
E2E Test
Unit Test
Integration Test

Backend Test Pyramid
E2E Test
Unit Test
Integration Test

Frontend Test Pyramid
Unit Test
E2E-Test
Integration Test

•Backend (Java) testing is well
established
•The important stuff is in the backend!
Some enterprise architect
•Need for unit testing
•And tooling is quite nice tbh
•mvn clean install ftw
In the backend

In the frontend
•Frontend (JS) testing used to be an
afterthought
•Less (underrated!) complexity
•Nah! We don’t need to test that
The same enterprise architect
•Messy tooling
•Tons of config and dependencies
needed to be orchestrated
•And that rumor is still in our heads

•In the age of SPAs
•More code than in the past
•Complex code
•Frontend code in itself is complex
•Also: Compared to backends
•Need for testing is already there
•Selenium is not enough!
Proper Testing in the frontend is important

Two sided test pyramid
E2E Test
Unit Test
Integration Test
Unit Test
Integration Test
Frontend Backend

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A library that puts the fun in testing
Enter Jest

Delightful JavaScript Testing
https://facebook.github.io/jest/
•Testing library/framework
•Comparable to
•JUnit (Java)
•Test::Unit (Ruby)
•PHPUnit (PHP)
What is Jest?

•Developed by Facebook
•Current version 23
•MIT licensed
•> 18.000 Github Stars
•Has been around since 2014
Jest: Facts and figures

•Comes from Facebook but it’s not
limited to React
•Works like a charm in my projects
•React based
•Vue based
•JQuery backed
Jest: Facts and figures

Easy Setup
You can start with
very little config
Awesome CLI
A really polished
product
All in the box
Little to no need for
third party libs
Why I like Jest

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Simple to install and simple to use
Easy setup

Create-React-App
Probably the easiest way to get going is
to just use create-react-app.
repl.it
Use an online repl might be even easier.
npm install jest
Come on, let’s get our hands dirty!
Let’s get started
I want to write the first test!

Be amazed
Install dependencies $ npm install -D jest
Let’s get started
package.json
"scripts": {
"test": "jest",
"test:watch": "jest --watch"
}
Write a simple test
describe('Demo should', () => {
it('be green', () => {
expect(true).toBe(true);
});
});
Run it $> npm test
PASS ./demo.spec.js
Demo should
✓ be green (2ms)
Test Suites: 1 passed, 1 total
Tests: 1 passed, 1 total
Snapshots: 0 total
Time: 1.606s
Ran all test suites.

Anatomy of a test
describe("add", () => {
beforeEach(() => {})
afterEach(() => {})
beforeAll(() => {})
afterAll(() => {})
it("should compute the sum", () => {
expect(add(1, 2)).toBe(3);
});
});
test("should compute the sum", () => {
expect(add(1, 2)).toBe(3);
})
This gives a nice descriptive scoped
structure of your test.
As a bonus, Jest can print the
results nicely,
I can recommend that structure.
A more traditional way of writing
your tests. Possible, but less
expressive than the describe style.

Best practice: Nested blocks
•Use nesting of describe blocks to group your tests logically
•Setup code can be fine tuned for specific case
•Setup in nested describe
describe('Customer', () => {
beforeEach(() => {});
describe('placeOrder should', () => {
it('be valid', () => {
expect(customer.placeOrder()).toBe(true);
});
});
describe('checkQty should', () => {
it('be valid', () => {
expect(customer.checkQty()).toBe(1);
});
});
});
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Useful shortcuts
fdescribe()/describe.only()
fit()/it.only()
ftest()/test.only()
Executes only this block (in the block)
Skips the block
xdescribe()/describe.skip()
xit()/it.skip()
xtest()/test.skip()

Where are my tests?
•Out of the box, Jest looks for
__tests__/*
bar.spec.js
foo.test.js
•Configurable
•Suited my needs well thus far

Best practice: Code and Test nearby
•Makes it way easier to navigate between code and test
•Unusual at first, but grow fond of it
add.js
add.spec.js
mockable.js
mockable.spec.js
objectProducer.js
objectProducer.spec.js
stringProducer.js
stringProducer.spec.js
src/myApp

There is more
•Works with ES6
•With Babel
•Compile to JS friendly
•e.g. works (now) well with TypeScript
•All presets are configurable
•package.json
•CLI
•JS

Test runner trivia
•Jest runs
•tests in parallel
•tests in a sandbox
•No conflicting of tests
•failed tests first

Runs in your CI Server
I work with it in:
•Travis
•Gitlab CI
•Jenkins

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Fun to work with
Awesome CLI

PASS client/service/skippingAndForcing.spec.js
PASS client/service/customMatchers.spec.js
PASS client/service/asymetricExpectations.spec.js
PASS client/service/serviceUser.spec.js
PASS client/service/newExpectations.spec.js
PASS client/service/httpServiceWithPromises.spec.js
PASS client/components/FirstTest.spec.js
PASS client/components/TextComponent.spec.jsx
PASS client/components/App.spec.jsx (5.539s)
PASS client/components/ListComponent.spec.jsx (5.589s)
Tests: 5 skipped, 23 passed, 28 total
Snapshots: 5 passed, 5 total
Time: 9.83s
Test results
•Nice output out of the box

FAIL ./add.spec.js
● add › should compute the sum
expect(received).toBe(expected) // Object.is equality
Expected value to be:
4
Received:
3
8 |
9 | it('should compute the sum', () => {
> 10 | expect(add(1, 2)).toBe(4);
11 | });
12 | });
13 |
at Object.<anonymous> (add.spec.js:10:27)
PASS ./asymetricExpectations.spec.js
..
Test Suites: 1 failed, 8 passed, 9 total
Tests: 1 failed, 5 skipped, 15 passed, 21 total
Meaningful error reports

Watch mode
•Looks for changes in the background
•Executes test automatically
•And more!
•Start via:
$ jest --watch

Best practice: Watch mode on
•You get very fast feedback
•Nice for TDD
•Immersive
•One less task to do manually
•Make it a habit

Code coverage
$ jest --coverage
PASS client/service/newExpectations.spec.js
-----------------------------|----------|----------|----------|----------|----------------|
File | % Stmts | % Branch | % Funcs | % Lines |Uncovered Lines |
-----------------------------|----------|----------|----------|----------|----------------|
All files | 90.91 | 100 | 86.67 | 93.33 | |
components | 100 | 100 | 100 | 100 | |
App.jsx | 100 | 100 | 100 | 100 | |
ListComponent.jsx | 100 | 100 | 100 | 100 | |
TextComponent.jsx | 100 | 100 | 100 | 100 | |
service | 81.25 | 100 | 77.78 | 84.62 | |
httpService.js | 100 | 100 | 100 | 100 | |
httpServiceWithPromises.js | 100 | 100 | 100 | 100 | |
serviceUser.js | 62.5 | 100 | 50 | 66.67 | 9,10 |
-----------------------------|----------|----------|----------|----------|----------------|
•Computes coverage out of the box

Best practice: Coverage is your friend
•Sane default out of the box
•I use it regularly to see, where some test love may be needed
•Don’t go overboard with the numbers
•Use it as a guideline
•CI is a good place to make this visible

$ jest --verbose
PASS client/components/App.spec.jsx
App
✓ renders (14ms)
✓ calling Service correctly (3ms)
PASS client/service/httpServiceWithPromises.spec.js
httpService getPosts should
✓ talk to the backend using done (1ms)
✓ talk to the backend using promise returns
✓ talk to the backend using async await (1ms)
httpService getPostsRejecting should
✓ reject using done
✓ reject using expectations
✓ reject using async await (1ms)
Tests: 5 skipped, 23 passed, 28 total
Snapshots: 5 passed, 5 total
Time: 2.187s
Even nicer output
•verbose option

IDE Support
It just works
•I use it mainly in IntelliJ/Webstorm and it just works
•Use a Run Config like any other test
•Works fine in VSCode
•Some more config
•Not as polished as IntelliJ
•But fine for me
•I use the Jest extension
•I often use it on the command line
•Alongside the IDE
•The watch feature shines there

3
Less need for 3rd party libs
Batteries included

Expectations
Mocking
Snapshot Tests
What’s in the box
Async Tests
DOM Testing
Code coverage

Expectations .toBe(value)
.toHaveBeenCalled()
.toBeCloseTo(number, numDigits)
.toBeDefined()
.toBeFalsy()
.toBeGreaterThan(number)
.toBeGreaterThanOrEqual(number)
.toBeLessThan(number)
.toBeLessThanOrEqual(number)
.toBeInstanceOf(Class)
.toBeNull()
.toBeTruthy()
.toBeUndefined()
.toContain(item)
.toContainEqual(item)
.toEqual(value)
.toHaveLength(number)
.toMatch(regexpOrString)
.toMatchObject(object)
…
•No need for separate Mocha/Jasmine/expect
•All the expectations you need
•Matchers are extendable
https://facebook.github.io/jest/docs/en/expect.html

Expectations: Examples
Expecting Objects
1 describe('createCustomer should’, () => {
2 it('produce a valid customer',() => {
3 const customer = {name: 'Peter', premium: true};
4 expect(customer).toBeDefined();
5 expect(customer.name).toEqual('Peter');
6 expect(customer.name).toEqual(expect.stringContaining('ete'));
7 expect(customer).toEqual(expect.objectContaining({premium: true}));
8 })
9 });
Expecting Arrays
describe('createCustomers should', () => {
it('work with many customer',() => {
const customers = [
{name: 'Peter', premium: true},
{name: 'Max', premium: false},
{name: 'Tine', premium: true}
];
expect(customers).toContainEqual({name: 'Tine', premium: true});
})
});
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Best practice: Few expectations
•Don’t go overboard with the expectations in the it blocks
•In doubt write one more it block
•Expectations following a failing expect don’t execute

Mocking
In a unit test it is often unfeasible to kickstart your whole dependency graph
Mocking lets you narrow down the scope of your test
{
}{
}

Mocking
•No need for separate Sinon.js
•Works similar to Mockito in Java
Mock Functions
Lets you replace more
complex logic with logic right
for your test
Manual Mocks
Lets you replace whole
modules for your test

Mock functions
describe('placeOrder should', () =>
it('order an available product', () => {
const product = {
isAvailable: jest.fn().mockReturnValue(true),
order: jest.fn()
};
placeOrder(product, 2, 'SOMMER');
expect(product.order).toHaveBeenCalled();
expect(product.order).toHaveBeenCalledTimes(1);
expect(product.order).lastCalledWith(2);
expect(product.order).toHaveBeenCalledWith(2);
})
);
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export function placeOrder(product, qty, seasonId) {
if (product.isAvailable(seasonId)) {
product.order(qty);
}
}
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Best practice: Mock to reduce scope
•Use mocking if
•you set things up that don’t belong to your test
•you are only interested in the result (or an error!) of a dependency
•This makes the test more readable and potentially faster
•But beware if you see too much mocking involved
•Maybe your test can be smaller
•Maybe you need to restructure your dependencies

Snapshot tests
expect(validationMessage('Vorname')).toMatchSnapshot();
•Not (strictly) UI related!
•Jest stores result of function (i.e any serializable thing) to the fs the first run
•The stored thing is called Snapshot
exports[`validationMesssage should produce a valid message 1`] =
`"Feld 'Vorname' ist ein Pflichtfeld."`;
•Compares to this in consecutive test runs
expect(value).toMatchSnapshot()
Received value does not match stored snapshot 1.
- "Feld 'Vorname' ist ein Pflichtfeld."
+ "Das Feld 'Vorname' ist ein Pflichtfeld.“
› 1 snapshot test failed.

Test that would require some effort to expect
React components (that should not change)
Things you would not test otherwise
Best practice: Use Cases for
snapshot tests

Snapshot test may be brittle
You NEED to review your snapshot changes
No jest -u fire and forget
Prefer unit tests
Best practice: Caveats of snapshot
tests

it("talk to the backend using done", done => {
getCustomers().then(data => {
expect(data).toEqual(TEST_RESPONSE);
done();
});
});
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Async tests
•You’ll encounter async code quite frequently
•Testing using callback way still works well
•Known from Jasmine

Async tests
it("talk to the backend using promise returns", () => {
return getCustomers().then(data => {
expect(data).toEqual(TEST_RESPONSE);
});
});
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•Return a promise
it("talk to the backend using expectations", () => {
expect(getCustomers()).resolves.toEqual(TEST_RESPONSE);
});
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•Use a matcher to resolve the provided promise
it("talk to the backend using async await", async () => {
const customers = await getCustomers();
expect(customers).toEqual(TEST_RESPONSE);
});
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•Use async/await

DOM testing
•Jest comes with JSDOM to support testing DOM.
•JSDOM simulates a DOM-enviroment.
it('show text on page', () => {
const text = document.querySelector('#myText').innerHTML;
expect(text).toEqual('Lorem');
});
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const setupDOM = () => {
document.body.innerHTML =
'<div>' +
' <div id="myText">Lorem</div>' +
'</div>';
};
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•In a test you can setup a mock structure of your sites markup
•Assert in the DOM
•Useful when in conjunction with e.g. JQuery

Best practice: Keep away from DOM
•As long as you can
•Tests will be slow and brittle
•If you must expect sth in the DOM, keep it brief

DOM testing (outlook)
Test the rendering of your React application:
Enzyme (http://airbnb.io/enzyme/)
Enzyme is a JavaScript Testing utility for React that makes it
easier to assert, manipulate, and traverse your React
Components' output.
Browser testing using Jest:
Puppeteer (https://github.com/GoogleChrome/puppeteer) and
its Jest integration (https://facebook.github.io/jest/docs/en/
puppeteer.html)
Puppeteer is a Node library which provides a high-level API to
control headless Chrome or Chromium over the DevTools
Protocol.

Expectations
Mocking
Snapshot Tests
What’s in the box
Async Tests
DOM Testing
Code Coverage
And more

53
Some closing words
Coming to an end

What we had to do
{
"presets": ["env"]
}
Install dependencies $ npm install -D jest
package.json
"scripts": {
"test": "jest",
"test:watch": "jest --watch"
}
babel
//.babelrc
{
"presets": ["env"]
}
$ npm install -D babel-jest babel-core
babel-preset-env

Easy to use and setup
Powerful features set yet familiar
Polished product (even the CLI)
Try Jest yourself

facebook.github.io/jest/
Some links
Tryout Jest
Release Blog
facebook.github.io/jest/blog/
Curated Links
github.com/jest-community/awesome-jest
Sample Code
github.com/holgergp/jestStarter

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