NPM (Node Package Manager) is the package manager for JavaScript. It lets you install, share, and manage code libraries (called packages) that other people have written.

Instead of writing everything from scratch, you install a package:

npm install express

Now you have a web framework. Someone else wrote it, tested it, and maintains it. You just use it.

What’s a package?

A package is a reusable piece of code published to the npm registry (npmjs.com). There are over 2 million packages. Examples:

• express — web framework
• react — UI library
• lodash — utility functions
• axios — HTTP client
• dotenv — load .env files

Key files

package.json — your project’s manifest. Lists dependencies, scripts, and metadata:

{
  "name": "my-app",
  "dependencies": {
    "express": "^4.18.0"
  },
  "devDependencies": {
    "jest": "^29.0.0"
  },
  "scripts": {
    "start": "node index.js",
    "test": "jest"
  }
}

package-lock.json — locks exact versions so every developer and CI server gets identical dependencies. Always commit this file.

node_modules/ — where packages are actually installed. Never commit this (add to .gitignore).

Essential commands

npm init -y              # Create package.json
npm install express      # Add a dependency
npm install jest -D      # Add a dev dependency
npm install              # Install all deps from package.json
npm run start            # Run the "start" script
npm run test             # Run the "test" script
npm update               # Update packages to latest allowed
npm outdated             # Show outdated packages
npm uninstall express    # Remove a package

dependencies vs. devDependencies

• dependencies — needed to run your app (express, react)
• devDependencies — only needed during development (jest, eslint, typescript)

npm install express        # → dependencies
npm install jest -D        # → devDependencies

npx — run without installing

npx create-react-app my-app   # Runs the package without installing globally
npx prettier --write .         # Run prettier once

NPM vs. alternatives

Tool	Speed	Lock file	Disk usage
npm	Good	package-lock.json	Normal
pnpm	Fastest	pnpm-lock.yaml	Smallest (shared store)
yarn	Fast	yarn.lock	Normal
bun	Fastest	bun.lockb	Normal

All use the same npm registry. You can switch between them freely.

For the full command reference, see the npm cheat sheet and pnpm cheat sheet.