Getting Started with Jest

Unit Testing and its Importance

Testing is an integral part of every software development process, and there are different levels on which the given software undergoes testing.

So as the name suggests, unit testing is a type of software testing that involves testing each software application unit. This Component testing guarantees that every unit of the software product performs expectedly, like functional correctness. Our function under test gives output as expected, memory and speed constraints, meaning the unit also satisfies the given limits of memory and speed requirements.

Unit testing is the first and building block of the testing process. Test engineers will start checking every application module independently or one by one. This process is known as Unit testing, also known as components testing.

Below are some of the crucial reasons for unit testing:

• Unit Testing is used to test specific units or modules of the software, in turn, to test or validate the correctness of the code, performance of the code, code coverage, implementation, and maintenance of coding standards.
• This test simplifies the debugging process since It also enforces modularity, i.e., unit tests are run on individual units or components, which means that the code is aptly divided into modules. Even if a test or a test suite fails, the developer knows which part of the code needs to be debugged.
• Skipping or limiting unit tests can lead to increased defects and testing costs as it is difficult to remove the bugs later.
•  At the time of writing, a unit test developer inadvertently writes the expected functionality of code which serves the purpose of detailed documentation that explains the working of the code and helps in an easier understanding of functionality in the future.
• It helps with code reusability by migrating code and test cases. 
• Codes are more reliable since they went through a rigorous unit testing phase that has been checked on different aspects.

Going through these points, it is highly evident that Unit Testing is an important part and parcel of the testing process and the entire software development life cycle.

Use Jest for Unit Testing

 Jest is a popular unit testing framework for testing JavaScript-based codebases. Testing the front-end requires extensive and time-consuming configuration, Jest works like magic here and reduces this complexity to a great extent. 

Javascript testing frameworks is one of the most popular unit testing frameworks in existence, it can validate almost everything built on JavaScript, especially browser rendering, the Jest is also widely preferred for automated browser testing.

We can quickly add unit tests to JavaScript apps with the Jest test runner. It's effortless to get running, and we can write lots of tests with it that run quickly. Additionally, Jest provides a blended package of an assertion library along with a test runner and a built-in mocking library. It stands out by virtue of its simplicity, making it an ideal tool to test JavaScript Library Projects such as AngularJS, Vue JS, Node JS, Babel, and TypeScript.

Advantages of Jest

Jest is no one framework when it comes to unit testing frameworks; this is due to its advantages which I have listed below:-

• Fast, safe, and efficient - Jest framework is swift, secure, and efficient when executing test cases.
• Code coverage - Jest provides great code coverage; code coverage is what percentage of the code is currently covered by unit tests.
• Easy mocking: It is one of the most painful things to do for testing engineers, Jest provides a very easy way to mock functions, modules, etc.
• Easy to set up and configure: we can install Jest as a dependency and start writing tests with minimal adjustments or configurations. 
• Command Line Tool: Jest also provides a CLI tool that helps to use Jest from the command line.
• Jest provides a way to debug tests using .only and .skip individually to test a single test or skip a test.
• Isolated tests: Jest tests don't influence each other results; all these tests are executed in parallel Jest collects the results from all the test processes.
• Snapshots testing: to test the front-end, snapshot provides a way to verify the correctness of large objects. This means we don't have to check each and every property present in an object Jest works like magic to test snapshots. 
• Rich API: Jest offers many specific assertion types for particular needs. Besides that, its excellent documentation should help you get started quickly.
• Pre-commit hooks – Jest provides a feature called pre-commit hooks to check test cases that have been changed in the previous commit or that are significant in the current run, 
• Easy to Migrate – thanks to the code mods module, we can migrate any project to use the Jest framework without any issue or code interaction.
•  Apart from primary test runner, Jest also provides auto mock modules, setup coverage thresholds, module mapper, etc.       

It is now evident why Jest is a widely used unit testing framework. 

Installing Jest

First of all, we have to make sure that nodeJs is installed in your computer, in the command prompt run the following command to check the version of nodeJs, if nodeJs is installed in your computer, something like v16.13.2 will appear otherwise an error will be shown.

node -v

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Now, we have to initialize a node project, to do that run the following command in your IDE to create a node module.

npm init -y

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Now to install Jest run the following command, we can use either npm or yarn.

npm install --save-dev jest

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Or,

yarn add --dev jest

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One simple configuration is left to do, in package.json, under scripts property, add jest as a test script, like shown below.

"scripts": {
    "test": "jest"
  },

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With only these simple steps, we are now ready to write our first Jest unit test.

Writing first unit test

Let’s first write some functions to test, create function.js in the same folder as package.json and write some functions to test.

Program

// creating square function
function square(x) {
    return x*x;
  }
  module.exports = square;

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Now to write our test scripts, create a new test file named function.test.js, Jest will automatically understand that this is a test file.

Test

// Importing square function.
const square = require('./function');

// writing test.
test('square of 5 is 25', () => {
  expect(square(5)).toBe(25);
});

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Run Code

Run npm test in IDE to execute your test code.

Output

Congratulations! We have successfully tested our first unit test using Jest.

FAQs

1. How do I get Started with Jest?
   Install node into your machine, create a new node project using npm init -y, do a minimal configuration, and you are ready to write your unit tests using Jest.
    
2. Do you need to import Jest?
   In your test files, Jest puts each of these methods and objects into the global environment. You don't have to require or import anything to use them.
    
3. What is a jest test runner?
   Jest is a JavaScript test runner, a JavaScript library for creating, running, and structuring tests. Jest ships as an NPM package; you can install it in any JavaScript project.
    
4. Do I need to install Jest globally?
   We can install Jest locally as well as globally, to install Jest globally, sudo authorization is required, run npm install --save-dev jest -g, and access from the terminal with the command jest.

Key Takeaways

In conclusion, Jest is an excellent framework for unit testing; it is instrumental in writing easy, fast, and efficient unit tests with great code coverage and many other functionalities. 

Hence learning never stops, and there is a lot more to learn. So head over to our practice platform Coding Ninjas Studio to practice top problems, attempt mock tests, read interview experiences, and much more.!

Happy Learning!