It’s not enough to simply write lines of logic in the world of coding. Consider this: You’ve created a brilliant piece of software that appears to work flawlessly. But how can you be sure it’ll continue to work, especially if you make changes?

This is where unit testing comes into play. It’s not about making coding more complicated. It’s about making it more stable and dependable. In this article, we’ll review the fundamentals of unit testing in JavaScript.

We’ll begin by explaining why unit testing is so important, and why you should never skip it. Then we’ll walk you through some easy-to-use JavaScript testing frameworks. Along the way, we’ll discuss common challenges and best practices for making your testing journey a success.

By the end of this article, you’ll realize that it’s not just about writing code, but also about writing code that holds up over time.

What is unit testing?

Unit testing is like running a reliability test on your code. A “unit” in coding is the smallest testable part of your application, whether it’s a function, method, or module.

Consider building a car: before you hit the road, you want to ensure that every wheel, engine, and essential component is in perfect working order, right? That checkpoint is unit testing. Unit testing provides instant feedback, allowing you to address issues as they arise and streamline the development process.

Unit testing also serves as a safety net, preventing the common scenario of fixing one bug and unintentionally introducing another. When you add new features or make changes, your tests ensure that existing functionalities don’t break, which is like having eyes on the back of your code’s head.

In essence, unit testing ensures that your software behaves as expected, catches problems early on, and maintains the strength and reliability of your codebase.

Unit testing also serves as a safety net, preventing the common scenario of fixing one bug and unintentionally introducing another.

Importance of unit testing

Unit testing is an essential practice with several key advantages:

• Provides early bug detection and cost savings: Unit testing serves as a vigilant gatekeeper, catching bugs as they emerge. Detecting and addressing issues early in the development process reduces the cost of fixing complex bugs later. It’s an investment in your codebase’s long-term stability and maintainability.
• Enhances code quality: Unit testing encourages developers to write modular and loosely coupled code. This, in turn, encourages good coding practices and improves overall code quality. Writing testable code often leads to more deliberate design and architecture decisions.
• Increases code confidence and Continuous Integration: Unit tests act as a layer of protection for developers, allowing them to make changes with confidence. A robust suite of unit tests ensures that new code doesn’t disrupt existing functionalities in continuous integration environments where code changes are frequently integrated.
• Eliminates fear of breaking existing functionalities: The fear of breaking existing functionalities frequently stymies refactoring. Unit tests alleviate this anxiety by acting as a safety net. Developers can refactor and optimize code without fear of unintended consequences.
• Develops documentation: Unit tests serve as executable documentation. They not only describe how each part of your code should behave, but they also show it in action. This living documentation is invaluable for onboarding new team members and comprehending the codebase’s complexities.
• Streamlines continuous delivery: Unit testing is a critical component of continuous delivery pipelines. Automated tests, including unit tests, ensure that changes can be deployed to production quickly and reliably. This speeding up of the delivery process corresponds to the needs of modern software development.
• Empowers collaboration: Unit tests serve as a contract between different parts of the codebase, empowering collaboration among development teams. When multiple developers work on different components, having unit tests ensures each component behaves as expected.
• Improves customer satisfaction and user experience: Dependable software equals a positive user experience. Unit testing contributes to the creation of robust and dependable applications by catching and fixing bugs early, ultimately increasing customer satisfaction.

In essence, unit testing is an important factor in the overall health and success of your software projects, not just a formality.

Unit testing frameworks

1. Jest

Jest is a JavaScript testing framework created by Facebook. It’s commonly used for testing JavaScript code, including React apps. Jest is well known for its speed and simplicity, making it a popular choice among developers.

• Jest requires minimal setup. It includes sensible defaults and requires no additional configuration, making it simple to begin testing.
• Jest is designed to be fast and can run tests in parallel, which optimizes test execution time. This is especially useful for large projects with large test suites.
• Jest introduces the concept of snapshot testing, in which it captures a component’s output and compares it to a previously saved snapshot. This aids in detecting unexpected changes in the UI.
• Jest includes mocking support, which enables you to easily simulate components or functions, making it easier to isolate and test specific parts of your code.

How Jest works

Step 1: Install Jest

To begin, use npm or yarn to install Jest into your project:

npm install –save-dev jest

Step 2: Update the package.json file

Once installed, we must instruct Jest on how to run your tests. This can be accomplished by including a “test” script in our package.json file:

“scripts”: {

“test”: “jest”

}

Step 3: Create a Jest test file. Assume we have a simple JavaScript function in a file called add.js:

// add.js

function add(a, b) {

return a + b;

}

module.exports = add;

Jest is used for testing to ensure that this function works as expected. The real magic happens in a corresponding test file: add.test.js:

// add.test.js

const add = require(‘./add’);

test(‘adds 4 + 6 to equal 10’, () => {

const num1 = 4;

const num2 = 6;

const result = add(num1, num2);

// Assert

expect(result).toBe(10);

});

To see the results, we type npm test in the terminal:

>> npm test

> test@1.0.0 test

> jest

PASS ./add.test.js

√ adds 4 + 6 to equal 10 (10 ms)

Test Suites: 1 passed, 1 total

Tests: 1 passed, 1 total

Snapshots: 0 total

Time: 0.869 s, estimated 2 s

Ran all test suites.

In this test, we’re telling Jest to see if the add function correctly adds 4 and 6 to yield 10.

Jest discovers and runs our test file when it’s executed. We use expect within the test function to assert the output of our add function. The toBe matcher, a Jest built-in feature, ensures that adding 4 and 6 exactly equals 10.

2. Mocha

Mocha is a powerful JavaScript testing framework that works in both the browser and Node.js environments. It has a robust set of features for testing asynchronous code, making it appropriate for a wide range of testing scenarios.

• Mocha supports a variety of assertion libraries and can be combined with other libraries or tools for mocking, spying, and code coverage.
• Mocha excels at testing asynchronous code by providing native support for async and await functions. This is essential when testing modern JavaScript applications that rely heavily on asynchronous operations.
• Mocha can run tests in both browser and Node.js environments, giving you the flexibility to test different parts of your application.
• Mocha includes several built-in reporters for displaying test results and support for custom reporters. This facilitates integration with various CI/CD tools.

How Mocha works:

Step 1: Install Mocha

To begin, we’ll install Mocha into our project:

npm install –save-dev mocha

Step 2: Update the package.json file

Configure the script to run all files that end spec.js:

“scripts”: {

“test”: “mocha *spec.js”

}

Step 3: Create our Mocha test file. Assume we have a function in the sum.js file that adds two numbers:

const sum = (a, b) => a + b;

module.exports = sum;

Now, we’ll create our corresponding test file, sum.spec.js, where we’ll test our function:

const sum = require(‘./sum’);

const assert = require(‘assert’)

describe(‘Sum Test’, ()=> {

it(‘Test if the sum of 3 and 4 will give 7’, ()=> {

assert.strictEqual(sum(3, 4), 7)

})

});

The above code is testing a sum function. The test asserts that the result of summing 3 and 4 should be strictly equal to 7 using assert.strictEqual.

And this will be the output:

>> npm test

> test@1.0.0 test

> mocha *spec.js

Sum Test

✔ Test if the sum of 3 and 4 will give 7

1 passing (18ms)

3. QUnit

QUnit is a powerful and user-friendly JavaScript testing framework. It was originally created by the jQuery team and has since evolved into a standalone testing framework. QUnit is intended to be simple yet versatile, making it appropriate for a wide variety of testing scenarios.

• QUnit is simple to set up and use, making it an excellent choice for projects that prioritize simplicity.
• Despite being standalone, QUnit seamlessly integrates with jQuery, making it an excellent choice for testing jQuery-based applications.
• Because QUnit includes support for testing asynchronous code, it’s well suited for applications that rely heavily on asynchronous operations.
• QUnit supports modular testing, which enables developers to organize and run tests for specific modules or components.

How QUnit works:

Step 1: Install QUnit and QUnit CLI

npm install –save-dev qunit

npm install –save-dev qunit-cli

Step 2: Create a QUnit test file. Let’s say we have a file named sum.js with a simple function:

function sum(a, b) {

return a + b;

}

Now, let’s create a QUnit test file, sum-test.js:

const sum = require(‘./sum’);

QUnit.module(‘sum’);

QUnit.test(‘Sum of 3 and 4 should be 7’, assert => {

assert.equal(sum(3, 4), 7);

});

Step 3: Run QUnit tests from the command line We can use qunit-cli to run our tests.

Add a script to our package.json:

“scripts”: {

“test”: “qunit-cli sum-test.js”

}

Now, we can run our tests using:

npm test

This will execute QUnit, run the specified test file, and give the result below:

>> npm test

> test@1.0.0 test

> qunit-cli sum-test.js

join

✔ Sum of 3 and 4 should be 7

Tests completed in 4 milliseconds.

1 tests of 1 passed.

Don’t forget to adjust the file paths in the script according to your project structure.

For browser testing, visit the QUnit website to explore guides and documentation on running tests in a browser environment.

Challenges to unit testing

While unit testing is extremely beneficial, it’s not without its challenges. Let’s look at some of the most common roadblocks:

• Time and effort: Writing thorough unit tests takes time and effort. In fast-paced environments, developers may feel pressed to prioritize feature development over testing. Balancing these demands is a common challenge.
• Keeping tests up to date: As a codebase evolves, so must its unit tests. Keeping tests up to date with code changes and refactors can be difficult, and failing to do so can result in outdated or ineffective tests.
• Isolation of units: It can be difficult to ensure true isolation of units during testing. Dependencies between units may introduce test dependencies inadvertently, making it difficult to isolate and test individual components.
• Test data and environment configuration: Creating and managing appropriate test data, as well as configuring the test environment, can be time-consuming. It’s critical for meaningful test results to have a consistent and reliable testing environment.
• Balancing unit and integration tests: Deciding what to test at the unit level versus the integration level can be difficult. Finding the right balance is critical for effective testing while avoiding duplication of effort.
• Perceived investment: Teams or developers who are unfamiliar with unit testing may be resistant to its adoption due to the perceived learning curve or initial setup effort. Part of the challenge is persuading stakeholders of the long-term benefits.
• Misconceptions about code coverage: Achieving 100% code coverage doesn’t guarantee thorough testing. Developers may mistakenly believe that high coverage equals high quality, ignoring critical edge cases.
• Team collaboration: It can be difficult to maintain consistency in testing practices across a development team. Individual approaches or levels of expertise can impact the effectiveness of unit testing as a collective effort.

Best practices of unit testing

For you to experience the full benefits of unit testing, it’s important to implement best practices that increase the efficiency of your testing efforts. Here are some key practices to keep in mind:

• Create isolated tests: Make sure that each unit test is self-contained and doesn’t rely on the state or results of other tests. Isolation aids in the identification of problems and simplifies debugging.
• Use clear and descriptive test names: Create descriptive test names that convey the test’s purpose. A well-named test serves as documentation and makes identifying problems easier.
• Keep tests small and focused: Each test should focus on a specific behavior or functionality. Smaller, more focused tests are easier to understand and maintain, and they provide clearer insights into potential problems.
• Only test one concern at a time: Each test should concentrate on validating a single concern or behavior. Avoid testing multiple behaviors in a single test, because that makes determining the cause of failures more difficult.
• Use meaningful assertions: Select assertions that express the expected behavior clearly. This simplifies understanding of the test’s purpose and the conditions under which it fails.
• Refactor tests on a regular basis: As your codebase evolves, revisit and refactor your tests to keep them in sync with code changes. This ensures that tests remain relevant and effective.
• Manage assumptions and constraints: Clearly document any assumptions or constraints on which your tests are based. This assists other developers in comprehending the context and purpose of the tests.

JavaScript unit testing: Final thoughts

In conclusion, selecting the appropriate unit testing framework is a critical decision for JavaScript developers. Jest, Mocha, and QUnit each bring unique capabilities to the table.

Ultimately, the decision is influenced by project requirements, developer preferences, and the individual features that each framework brings to your testing ecosystem. Whether it’s Jest, Mocha, QUnit, or another tool, developers in the JavaScript ecosystem have a variety of options to meet their testing needs. The key is to explore, experiment, and select the framework that best matches the project’s goals and your team’s expertise.

This post was written by Vincent Chosen. Vincent is a web developer and technical writer. He has proficient knowledge in JavaScript, ReactJS, NextJS, React Native, Nodejs and Database. Aside from coding, Vincent loves playing chess and discussing tech related topics with other developers.