Introduction

CommonJS Modules are the traditional module system used in Node.js. Before the introduction of ES Modules (ESM), CommonJS was the standard way to organize JavaScript code into reusable files.

A CommonJS module uses:

module.exports to export code from a file.
require() to import code into another file.

Every JavaScript file in Node.js is treated as a separate module. This helps developers split applications into smaller, reusable components such as utility functions, configuration files, API services, database connections, and business logic.

Although modern projects increasingly use ES Modules, CommonJS is still widely used in many existing Node.js applications and libraries. Understanding CommonJS is essential because you will often encounter it in legacy codebases and popular npm packages.

For automation engineers, CommonJS modules help organize Playwright, Selenium, Cypress, and API automation projects by separating utilities, page objects, configuration files, and reusable functions.

In this tutorial, you’ll learn how CommonJS modules work and how to use them effectively.

What are CommonJS Modules?

A CommonJS module is a JavaScript file that exports values using module.exports and imports them using require().

Each file has its own private scope, helping avoid variable conflicts and improving code organization.

Why Use CommonJS Modules?

CommonJS modules help developers:

Organize code into separate files.
Reuse functionality.
Improve maintainability.
Reduce duplicate code.
Keep applications modular.
Simplify testing.
Manage large projects efficiently.

Basic Syntax

Export

module.exports = value;

Import

const variable =

    require("./file");

Exporting a Function

math.js

function add(a, b) {

    return a + b;

}

module.exports =

    add;

Importing a Function

app.js

const add =

    require("./math");

console.log(

    add(10, 20)

);

Sample Output

Exporting Multiple Functions

calculator.js

function add(a, b) {

    return a + b;

}

function multiply(a, b) {

    return a * b;

}

module.exports = {

    add,

    multiply

};

Importing Multiple Functions

const calculator =

    require("./calculator");

console.log(

    calculator.add(5, 5)

);

console.log(

    calculator.multiply(5, 5)

);

Sample Output

10
25

Exporting an Object

config.js

module.exports = {

    appName:

        "Automation Framework",

    version:

        "1.0"

};

Importing an Object

const config =

    require("./config");

console.log(

    config.appName

);

console.log(

    config.version

);

Sample Output

Automation Framework
1.0

Exporting a Class

user.js

class User {

    login() {

        console.log(

            "User logged in."

        );

    }

}

module.exports =

    User;

Importing a Class

const User =

    require("./user");

const user =

    new User();

user.login();

Sample Output

User logged in.

Automation Testing Examples

CommonJS modules help organize automation frameworks into reusable components.

Playwright Example

browser.js

function launchBrowser() {

    console.log(

        "Launching Playwright browser."

    );

}

module.exports =

    launchBrowser;

test.js

const launchBrowser =

    require("./browser");

launchBrowser();

Sample Output

Launching Playwright browser.

Selenium Example

login.js

function login() {

    console.log(

        "User logged in."

    );

}

module.exports =

    login;

test.js

const login =

    require("./login");

login();

Sample Output

User logged in.

Cypress Example

dashboard.js

function verifyDashboard() {

    console.log(

        "Dashboard verified."

    );

}

module.exports =

    verifyDashboard;

test.js

const verifyDashboard =

    require("./dashboard");

verifyDashboard();

Sample Output

Dashboard verified.

API Testing Example

api.js

function callApi() {

    console.log(

        "API request executed."

    );

}

module.exports =

    callApi;

test.js

const callApi =

    require("./api");

callApi();

Sample Output

API request executed.

Data-Driven Testing Example

data.js

module.exports =

    [

        "TC001",

        "TC002",

        "TC003"

    ];

test.js

const testCases =

    require("./data");

console.log(testCases);

Sample Output

[ 'TC001', 'TC002', 'TC003' ]

Real-World Uses of CommonJS Modules

CommonJS modules are commonly used for:

Utility functions.
Configuration files.
Database connections.
API services.
Authentication.
Logging utilities.
Validation helpers.
Page Object Models.
Test data management.
Report generation.

CommonJS vs ES Modules

Feature	CommonJS	ES Modules
Import	`require()`	`import`
Export	`module.exports`	`export`
Loading	Synchronous	Static and asynchronous-friendly
Browser Support	No	Yes
Standard	Node.js	ECMAScript
File Type	`.js`	`.js` or `.mjs`

Common Mistakes

Forgetting `module.exports`

If you do not export a value, it cannot be accessed by other modules.

Incorrect File Path

Provide the correct relative path when using require().

Example:

const math =

    require("./math");

Mixing CommonJS and ES Modules Incorrectly

Avoid using require() and import together in the same file unless you understand Node.js interoperability.

Best Practices

Keep each module focused on one responsibility.
Use meaningful file names.
Export only what is necessary.
Group related functions together.
Organize automation utilities into separate modules.
Avoid circular dependencies.
Prefer ES Modules for new projects, but understand CommonJS for existing applications.

Conclusion

CommonJS modules have been the foundation of Node.js development for many years and continue to power countless applications and libraries. By using module.exports and require(), developers can create modular, reusable, and maintainable applications.

For automation engineers, CommonJS modules provide a practical way to organize browser utilities, API helpers, configuration files, page objects, and test data. Although ES Modules are now the preferred standard for modern JavaScript, understanding CommonJS remains essential because many existing Node.js projects and npm packages still rely on it.

Frequently Asked Questions (FAQs)

What is a CommonJS module?

A CommonJS module is a JavaScript file that exports functionality using module.exports and imports it using require().

What is `module.exports`?

module.exports is used to make variables, functions, objects, or classes available to other files.

What does `require()` do?

require() imports code that has been exported from another CommonJS module.

Can I export multiple functions?

Yes. You can export an object containing multiple functions and access them after importing.

Why are CommonJS modules useful in automation testing?

They help organize automation projects by separating reusable utilities, page objects, API helpers, configuration files, and test data into modular files.

Key Takeaways

CommonJS is the traditional module system used in Node.js.
It uses module.exports to export code.
It uses require() to import code.
Each file is treated as an independent module.
Modules improve code organization and reusability.
You can export functions, objects, arrays, variables, and classes.
CommonJS is widely used in legacy Node.js projects and many npm packages.
ES Modules are recommended for new projects, but CommonJS knowledge is still essential.
CommonJS helps organize automation frameworks into reusable components.
Mastering CommonJS modules makes it easier to work with existing Node.js applications and libraries.

What are CommonJS Modules?

Why Use CommonJS Modules?

Basic Syntax

Export

Import

Exporting a Function

Importing a Function

Sample Output

Exporting Multiple Functions

Importing Multiple Functions

Sample Output

Exporting an Object

Importing an Object

Sample Output

Exporting a Class

Importing a Class

Sample Output

Automation Testing Examples

Playwright Example

Sample Output

Selenium Example

Sample Output

Cypress Example

Sample Output

API Testing Example

Sample Output

Data-Driven Testing Example

Sample Output

Real-World Uses of CommonJS Modules

CommonJS vs ES Modules

Common Mistakes

Forgetting module.exports

Incorrect File Path

Mixing CommonJS and ES Modules Incorrectly

Best Practices

Conclusion

Frequently Asked Questions (FAQs)

What is a CommonJS module?

What is module.exports?

What does require() do?

Can I export multiple functions?

Why are CommonJS modules useful in automation testing?

Key Takeaways

Forgetting `module.exports`

What is `module.exports`?

What does `require()` do?