npm, package.json & Module System | JS Mastery

npm & package.json

// npm init -y → creates package.json
// package.json controls your project:
{
  "name": "my-app",
  "version": "1.0.0",
  "type": "module",      // use ESM (import/export)
  "main": "index.js",
  "scripts": {
    "start": "node index.js",
    "dev": "node --watch index.js",
    "test": "jest"
  },
  "dependencies": {
    "express": "^4.18.0"   // production
  },
  "devDependencies": {
    "jest": "^29.0.0"      // development only
  }
}

// Commands:
// npm install express      — add dependency
// npm install -D jest      — add devDependency
// npm install              — install all from package.json
// npm run start            — run script
// npm uninstall express    — remove package
// npm list                 — list installed

CommonJS vs ES Modules

// CommonJS (default in Node.js without "type":"module"):
// Export:
module.exports = { add, PI };
// Import:
const { add, PI } = require("./math");
const express = require("express");

// ES Modules (with "type":"module" or .mjs):
// Export:
export const PI = 3.14;
export function add(a, b) { return a + b; }
export default class App {}

// Import (.js extension required in ESM!):
import App, { PI, add } from "./math.js";
import express from "express";

// Semver ranges:
"^4.18.0"  // >= 4.18.0 < 5.0.0 (minor updates OK)
"~4.18.0"  // >= 4.18.0 < 4.19.0 (patch updates only)
"4.18.0"   // exact version

// Always commit package-lock.json
// Never commit node_modules — add to .gitignore

⚡ Key Takeaways

npm init -y bootstraps a project — always start here
dependencies = runtime; devDependencies = build/test tools
CJS: require/module.exports; ESM: import/export
Semver: ^ = minor/patch updates, ~ = patch only, exact = no updates
package-lock.json ensures reproducible installs — always commit it

🎯 Practice Exercises

EXERCISE 1

Create a utils package: math.js with arithmetic functions, string.js with string helpers. Set up an index.js barrel file. Import everything into main.js.

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