Module Export & Require

Splitting code into multiple files keeps a Node.js project organized and maintainable. In this chapter, we'll explore how to share functionality between files using CommonJS (CJS) and ES Modules (ESM).

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CommonJS Modules (CJS)

Problem

You have two files : app.js and xyz.js— and want them to interact. How can app.js execute code or use functions from xyz.js?

Solution

Use require() to load one module into another and module.exports to expose functionality.

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1. Requiring a Module

app.js

const xyzModule = require('./xyz'); // Load xyz.js
console.log("Running app.js");

xyz.js

console.log("xyz.js module is being loaded");
// By default, nothing is exported here.

Running node app.js logs:

xyz.js module is being loaded
Running app.js

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2. Exporting a Single Function

sum.js

function calculateSum(a, b) {
  return a + b;
}
module.exports = calculateSum;

app.js

const sum = require('./sum'); // Load the sum module
console.log("Sum:", sum(5, 3)); // Sum: 8

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3. Exporting Multiple Items

sum.js

const x = "Exported value";
function calculateSum(a, b) { return a + b; }
module.exports = { x, calculateSum };

app.js

const { x, calculateSum } = require('./sum');
console.log(x);                  // Exported value
console.log(calculateSum(10, 5)); // 15

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4. Destructuring Imports (Common Pattern)

app.js

const { x, calculateSum } = require('./sum');

This cleanly pulls out only what you need.

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5. Folder Modules (index.js)

Structure:

project/
├─ app.js
└─ utils/
   ├─ index.js
   ├─ sum.js
   └─ multiply.js

utils/sum.js

function sum(a, b) {
  return a + b;
}
module.exports = sum;

utils/multiply.js

module.exports = (a, b) => a * b;

utils/index.js

const sum = require('./sum');
const multiply = require('./multiply');
module.exports = { sum, multiply };

app.js

const { sum, multiply } = require('./utils');
console.log(sum(2, 3));      // 5
console.log(multiply(2, 3)); // 6

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6. Requiring JSON & Built‑in Modules

data.json

{
  "name": "MyApp",
  "version": "1.0.0"
}

app.js

const config = require('./data.json');
console.log(config.name); // MyApp

const fs   = require('fs');
const path = require('path');

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ES Modules (ESM)

ESM is the modern JavaScript module system. To enable it in Node.js, add "type": "module" to your package.json or use the .mjs extension.

ESM Module

• Newer version of Node.

package.json

{
  "name": "my-app",
  "version": "1.0.0",
  "type": "module"
}

1. Exporting

mathUtils.js

export const PI = 3.14159;
export function add(a, b) { return a + b; }
export default 2.718;

2. Importing

main.js

import E, { PI, add } from './mathUtils.js';
console.log(PI);     // 3.14159
console.log(add(2,3)); // 5
console.log(E);      // 2.718

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Key Differences: CJS vs. ESM

Feature	CommonJS (CJS)	ES Modules (ESM)
Loading	Synchronous: require() blocks until the module is loaded	Asynchronous: import can load in parallel without blocking
Strict Mode	Non-strict by default	Always strict—enforces better parsing and error handling

Overall, ES Modules offer non‑blocking loading and built‑in strict mode, making your code more efficient and reliable.

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Additional Patterns

• Alternative CJS Exports

  module.exports.x = x;
  module.exports.calculateSum = calculateSum;
  

• Import All

  import * as utils from './utils.js';
  

• Default-only Export

  export default function() { /*…*/ }
  

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Recap

• CommonJS uses require() and module.exports.
• ES Modules use import and export.
• Organize with folders, index.js, or JSON imports.
• Choose CJS for legacy, ESM for modern codebases.