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Getting started with JEST: An Easy Guide to JavaScript Testing for Learners📘

· 6 min read

As I began integrating Jest into my Next JS project, I learned how powerful and user-friendly it is for creating dependable tests. This blog is a personal walkthrough of my testing journey — from installing Jest to writing first test cases, applying best practices, and understanding why testing is important.

Whether you are new to testing or just getting started with Jest, this blog will help you:

  • Understand what Jest is and why it is used.
  • Learn how to set it up quickly.
  • Write simple and effective test cases.
  • Follow testing best practices to maintain clean code.
  • Gain confidence in your codebase by catching bugs early.

Introduction to JEST🔬​

What is JEST ?​

Jest is a exciting framework for testing JavaScript. This works if you are using: Next.js, JavaScript, React , Node, Angular, TypeScript and many more in your projects!

It comes with a bundled, everything you need for testing:

  • 🧪 Test runner
  • 📣 Assertion library
  • 🧱 Built-in mocking
  • 📊 Code coverage support

Why Are We Using JEST ?🤔​

We use Jest to make sure that our code works in the right way it is supposed to. Writing tests helps us in project:

  • 🐞 Catch bugs early
  • 🔐 Improve confidence in the codebase
  • 💥 Avoids breaking features when making changes

Why JEST Over Other Tools ?⚖️​

Here are the few reasons why Jest is preferred in many JavaScript projects:

  • ✅ Zero configuration – works correctly
  • ⚡ Fast and parallel testing
  • 🔥 Snapshot testing support
  • 🧪 Built-in mocking features
  • 🛠️ Great for React and Next js apps
  • 📊 Have built-in code coverage feature

🎯 In short: Jest helps us test our code efficiently and reliably with minimal setup.

How I Started Using JEST in My Project 🚀​

Getting started with Jest in my Next JS project was simple and smooth. Here's how I integrated it step by step:

🛠️ Step 1: Installing Jest​

Install Jest as a development dependency:

(bash)
pnpm install --save-dev jest

OR

npm install --save-dev jest

OR

yarn add --dev jest

⚙️ Step 2: Adding Test Script​

Add import '@testing-library/jest-dom'; in jest.setup.js file. If file is not available after installation, then create the file and then add the above mentioned!

Next, add the following script to package.json file:

(json)
"scripts": {
  "test": "jest"
}

This lets you run all your test cases using:

pnpm test OR npm test OR yarn test

📂 Step 3: Creating First Test​

Organize test files based on type:

  • For unit tests, place the test file next to the module/component (.test.js in the same folder).

  • For integration tests, use a dedicated __tests__ folder at the project root level.

👉 In my project, I preferred organizing all my test files inside a central __tests__ folder at the root of the project.

🧩 Example​

// example.js
function sum(x, y) {
  return x + y;
}
module.exports = sum;
// example.test.js
const sum = require('./sum');

test('adds 1 + 2 to equal 3', () => {
  expect(sum(1, 2)).toBe(3);
});

Best Practices While Using JEST✅​

Here are some best practices I followed while working with Jest in my project:

  • 📁 Organize test files based on type:

    • For unit tests, place the test file next to the module/component (.test.js in the same folder).
    • For integration tests, use a dedicated __tests__ folder at the project root level.

  • 🧪 Use .test.js extension for test file clarity.

  • 🔍 Write a small, focused, and independent test cases to isolate functionality.

  • 🧼 Avoid using comments in test files — your test cases themselves should be self-explanatory.

  • 🚫 No warnings should be present in the test code — keep it clean and error-free.

  • 🧹 Remove unused imports — avoid dead code and keep files lightweight.

  • 🧼 Clean up after tests if needed — especially when using async or DOM-related code.

  • 🔁 Use beforeEach() and afterEach() for reusable setup/teardown logic. They make sure that every test runs independently, which contributes to the upkeep of a clean and reliable testing environment.

  • 📊 Enable code coverage reports to track test completeness:

    (bash) pnpm test -- --coverage Or npm test -- --coverage Or yarn test --coverage

  • 📝 Use descriptive names for your test cases to make results easy to understand.

  • 🔒 Mock external APIs/libraries using jest.mock() to avoid real-time dependencies.

-> Starting with Jest was quick, and following these best practices helped keep my test suite clean, efficient, and easy to maintain.

Why Did I Do This? What Was the Purpose ?🎯​

The main goal of using Jest in my project was to ensure that:

  • 🧪 My code works as expected — I wanted confidence that any function or component I wrote returned the correct output and behaved properly in different conditions.
  • 🚫 To catch bugs early — Automated tests help me find issues before they reach production.
  • ♻️ To safely refactor code — With tests in place, I could change or improve my code without worrying about breaking existing functionality.
  • 🚀 To maintain code quality — Jest made it easy to add tests for new features, encouraging a test-driven or test-first mindset.
  • 🤝 To improve team collaboration — Other developers could understand and rely on the test cases to know what each function or component is expected to do.
  • 📈 To track coverage — Using Jest's --coverage flag, I could measure how much of my code was tested and focus on improving gaps.

Overall, adding Jest helped increase reliability, confidence, and quality in my codebase — which was the purpose behind integrating it from the beginning.

Conclusion🏁​

Integrating Jest into my project turned out to be one of the most impactful decisions in terms of improving code quality, reliability, and developer confidence.

It allowed me to:

  • 🐞 Catch bugs early
  • 🛠️ Refactor code safely
  • 🤝 Collaborate better with the team
  • 🧼 Maintain a clean and tested codebase

By following best practices, organizing tests well, and using features like mocking and coverage reports, Jest made testing straightforward and more efficient.

Whether you are working on a large application or a small feature, adding tests with Jest helps ensure that your code not only works — but continues to work, even as it grows.

✅ In short: Jest made my project development process more easier, stable, testable, and professional — and I highly recommend this testing framework adding it to any modern JavaScript/TypeScript projects.

📚 For more information, visit the official Jest documentation:
👉 JEST

Thanks for reading! Hope this blog post helps you get started with Jest smoothly.
Feel free to share or refer it whenever you need a quick guide on setting up and using Jest. 🙌