How to Mock `dotenv` in Jest: A Senior Developer’s Guide

If you’ve ever used the dotenv library in a Node.js project, you know how useful it is for loading environment variables from a .env file. But what happens when you want to mock dotenv in Jest for unit testing?

In this article, I’ll walk you through the correct way to mock dotenv using Jest, explain why you might want to do this, and provide clean, production-quality examples.

📌 Why Mock `dotenv` in Jest?

When writing unit tests, especially for backend services, you often want to avoid loading real environment variables. Maybe you want to simulate different values for process.env, or just avoid any .env side effects altogether.

Mocking dotenv allows you to:

Prevent external dependencies from interfering with tests
Inject specific process.env values
Increase test speed and isolation

Let’s get to it.

🔧 How to Mock `dotenv` with Jest

1. Install Dependencies

If you haven't already:

npm install dotenv --save
npm install jest --save-dev

2. The Code Under Test

Assume this is your app code (config.js):

const dotenv = require('dotenv');
dotenv.config();

module.exports = {
  port: process.env.PORT || 3000,
};

3. Mocking `dotenv` in Jest

Create a test file config.test.js:

jest.mock('dotenv', () => ({
  config: jest.fn(() => {
    process.env.PORT = '5000'; // mock your env values here
    return { parsed: { PORT: '5000' } };
  }),
}));

const config = require('./config');

describe('Config Module', () => {
  it('should return mocked port value', () => {
    expect(config.port).toBe('5000');
  });
});

✅ What This Does:

Mocks the config() function from dotenv
Sets process.env.PORT to a test value
Avoids calling the real .env file on disk

🧠 Pro Tips for Senior Developers

Keep your .env mocking logic isolated per test file
Use beforeEach or afterEach to reset process.env if needed
Don’t mock dotenv globally unless you're writing integration tests with custom runners

🏁 Conclusion

Mocking dotenv in Jest is a clean and effective way to control your test environment and keep your unit tests predictable. By avoiding file system dependencies and setting process.env directly, you can write faster and more reliable tests.

How to Mock dotenv in Jest: A Senior Developer’s Guide

📌 Why Mock dotenv in Jest?

🔧 How to Mock dotenv with Jest