Jest - Handson.txt - Delightful JavaScript Testing Writing...

Delightful JavaScript Testing
Writing tests have high impact in application development. The application will
be stable and easy to maintain if the test results have few bugs.
Jest is a JavaScript unit testing framework, maintained by Facebook that lets
you define your unit tests.
Features of Jest
Mocking
Jest Allows to mock objects in test files.
Can mock specific objects or turn on automatic mocking with ***automock***,
which will mock every object.
Snapshot Testing
UI testing to become simplified as Jest can capture snapshots of React trees or
other serializable values.
Jest is Fast!
Runs tests parallelly in processes to minimize test runtime.
Runs previously failed tests first.
Automatically finds tests related to changed files.
Easy to set up JavaScript testing solution.
Provides Test isolation i.e. no two tests will ever conflict with each other.
Jest works well with other testing libraries (example Enzyme, Chai).
Jest is used by Leading companies to test web applications, node.js services,
mobile apps, and APIs.
Installing Jest
Install Jest using ***npm***:
npm install jest
Or via ***yarn***:
yarn add jest
Katacoda - A Playground for Jest
Katacoda is an interactive technical learning platform for software developers.
Here you can use Katacoda Node.js v6 Playground to practice Jesting (Javascript
Testing) by installing Jest in Katacoda Terminal.
First Test Using Jest
Create a sum.js file with a function that adds two numbers:
function sum(a, b) {
return a + b;}
module.exports = sum;
Create sum.test.js file. Let us get started by writing a test for the above
function:
const sum = require('./sum');
test('addition: 1+2=3', () =>{
expect(sum(1,2)).toBe(3);
});
Note: To create files, run the command touch <<filename>> in Katacoda terminal.
Run the Test
You can run Jest from the terminal and finally, you get test results.
Run all tests (default):
jest
Run only the tests that were stipulated with a pattern or filename:

jest sum.test --config=package.json
Jest runs all the files with .spec.js and .test.js extension in the current
directory.
Testing a To-do List Application
A simple To-do List Application has a lot of features to help you keep track of
your daily commitments - Anywhere. Anytime.
Let us test the javascript To-do List Application that has the following
functions:
addTask
updateTask
closeTask
deleteTask
***All hands-on across this course is based To-do list application***, to
explore your understanding of the concepts and writing tests.
You can refer the To-do List JavaScript functions in Codepen and can make use of
that when you try to write tests on your own.
-------------
Jest is used to test _____________. JavaScript
Jest works well with testing libraries like _________. All
Jest can be installed using ________. npm install --save-dev jest
Jest is a JavaScript unit testing framework. True
Jest is maintained by _________ . Facebook
-------------
Introduction to Matchers
Jest utilizes matchers to test values in different ways. There are several
matchers available and this section will introduce few of the most useful ones
like,
Common Matchers
Truthiness
Numbers
Strings
Arrays
Exceptions etc.