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Chapter 2: Module System

Node.js uses the CommonJS module system, where each file is a module. This chapter will introduce module creation, import, and export in detail.

Learning Objectives​

Through this chapter, you will master:

  • How the CommonJS module system works
  • Module creation and export
  • Module import and usage
  • ES6 module syntax
  • Module caching mechanism

CommonJS Module System​

Module Export​

// math.js
function add(a, b) {
  return a + b;
}

function subtract(a, b) {
  return a - b;
}

// Method 1: Export object
module.exports = {
  add,
  subtract
};

// Method 2: Direct function export
module.exports.add = add;
module.exports.subtract = subtract;

// Method 3: Using exports
exports.add = add;
exports.subtract = subtract;

Module Import​

// app.js
const math = require('./math');

console.log(math.add(5, 3)); // 8
console.log(math.subtract(5, 3)); // 2

// Destructuring import
const { add, subtract } = require('./math');
console.log(add(5, 3)); // 8

ES6 Module Syntax​

Export Syntax​

// math.mjs
export function add(a, b) {
  return a + b;
}

export function subtract(a, b) {
  return a - b;
}

// Default export
export default {
  add,
  subtract
};

Import Syntax​

// app.mjs
import { add, subtract } from './math.mjs';
import math from './math.mjs'; // Default import

console.log(add(5, 3)); // 8
console.log(math.add(5, 3)); // 8

Built-in Modules​

Node.js provides many built-in modules that can be used without installation.

Common Built-in Modules​

// File system
const fs = require('fs');

// Path handling
const path = require('path');

// HTTP server
const http = require('http');

// URL handling
const url = require('url');

// Operating system information
const os = require('os');

Module Caching​

Node.js caches loaded modules to improve performance.

// First load
const math1 = require('./math');

// Second load - returns cached module
const math2 = require('./math');

console.log(math1 === math2); // true

Practice Exercise​

Create a simple calculator module:

// calculator.js
class Calculator {
  add(a, b) {
    return a + b;
  }
  
  subtract(a, b) {
    return a - b;
  }
  
  multiply(a, b) {
    return a * b;
  }
  
  divide(a, b) {
    if (b === 0) {
      throw new Error('Division by zero is not allowed');
    }
    return a / b;
  }
}

module.exports = Calculator;
// main.js
const Calculator = require('./calculator');

const calc = new Calculator();

console.log(calc.add(10, 5)); // 15
console.log(calc.subtract(10, 5)); // 5
console.log(calc.multiply(10, 5)); // 50
console.log(calc.divide(10, 5)); // 2

Summary​

This chapter introduced Node.js module system, including:

  • CommonJS module import and export
  • ES6 module syntax
  • Built-in module usage
  • Module caching mechanism

Mastering the module system is fundamental to Node.js development, providing powerful support for code organization and reuse.

Next Steps​