Node, npm, and running scripts

So far, JavaScript has run in the browser. Node.js runs JavaScript on your machine — as a server, a build tool, or a standalone script. Understanding the runtime and its package ecosystem is essential for modern JavaScript development.

Node.js

Node.js is a JavaScript runtime built on Chrome’s V8 engine. It provides JavaScript access to the file system, network, environment variables, and other system-level APIs that browsers do not expose.

Check if Node is installed:

node --version

Run a JavaScript file:

node index.js

Run a one-liner:

node -e "console.log('Hello from Node')"

Open an interactive REPL:

node
> 2 + 2
4

package.json

package.json is a project’s manifest. It lists dependencies, defines scripts, and configures tooling:

{
  "name": "my-app",
  "version": "1.0.0",
  "type": "module",
  "scripts": {
    "dev": "node server.js",
    "build": "node build.js",
    "test": "node test.js"
  },
  "dependencies": {
    "express": "^4.18.0"
  },
  "devDependencies": {
    "eslint": "^8.0.0"
  }
}

• "type": "module" enables ES module syntax (import/export) in .js files
• dependencies are packages needed in production
• devDependencies are packages needed for development (linters, test runners, build tools)

Installing packages

Run scripts install packages from the npm registry:

# Install all dependencies
npm install

# Install a specific package
npm install express

# Install a dev dependency
npm install --save-dev eslint

The package is downloaded to node_modules/ and listed in package.json.

Running scripts

Scripts defined in package.json run with npm run:

npm run dev      # runs `node server.js`
npm run build    # runs `node build.js`
npm run test     # runs `node test.js`

Scripts in the scripts section have access to locally installed binaries in node_modules/.bin/. This means you can run tools installed as dev dependencies without installing them globally:

{
  "scripts": {
    "lint": "eslint src/",
    "format": "prettier --write src/"
  }
}

npm run lint     # runs node_modules/.bin/eslint
npm run format   # runs node_modules/.bin/prettier

npx runs one-off commands

npx runs a package binary without installing it globally:

npx http-server .    # serves current directory on a local port
npx create-vite      # scaffolds a new project

If the package is already installed locally, npx uses that version. Otherwise, it downloads and runs it temporarily.

Environment variables

Node.js exposes environment variables through process.env:

const port = process.env.PORT || 3000;
const apiKey = process.env.API_KEY;

console.log(`Server running on port ${port}`);

Set environment variables when running a script:

PORT=8080 node server.js        # Unix
$env:PORT = 8080; node server.js # PowerShell

What to carry forward

• Node.js runs JavaScript outside the browser with access to system APIs
• node file.js runs a JavaScript file
• package.json defines a project’s name, dependencies, and scripts
• "type": "module" enables import/export in .js files
• npm install installs dependencies; npm install package-name adds a new one
• npm run script-name runs scripts from package.json
• npx runs one-off commands without global installation
• environment variables are accessed via process.env

Tooling is the foundation of productive JavaScript development. The next lesson covers project structure in more depth.