Getting Started

JavaScript was born in the browser, but runtimes like Node.js have carried it to servers, command-line tools, build pipelines and desktop apps. This chapter shows the two places you will run code most often, then walks you through your first script, a new npm project and a comfortable editor setup.

Two runtimes, one language

The core language (variables, functions, arrays, Promises) behaves the same everywhere. What changes is the set of APIs around it: document exists only in the browser, while the fs module for reading files exists only in Node.js.

Try it in the browser console

The quickest way to run JavaScript needs no install at all. In Chrome, Edge or Firefox, press F12 or Ctrl+Shift+I (Cmd+Option+I on macOS) and open the Console tab. Type a line, press Enter, and the result appears immediately.

console.log("Hello, JavaScript!");

const scores = [72, 95, 88];
scores.sort((a, b) => a - b);
console.log(scores); // [72, 88, 95]

document.title; // the current page title (browser only)

The console echoes the value of the last expression, which makes it ideal for quick experiments. Anything you type is lost on reload, though, so move longer code into a file.

Install Node.js

Node.js runs JavaScript outside the browser. Download an installer from nodejs.org; unless you have a reason not to, pick the LTS (long-term support) release. If you need to switch between versions, a version manager such as nvm, fnm or Volta makes that painless.

npm, the package manager, ships with Node.js. Confirm both are on your path:

node --version
npm --version

Run your first script

Create hello.js in any folder. process.argv holds the command-line arguments; the values you pass start at index 2.

// hello.js
const name = process.argv[2] ?? "world";
const now = new Date().toLocaleTimeString("en-US");

console.log(`Hello, ${name}! It is ${now}.`);

Run it by handing the file to node:

node hello.js
node hello.js Ada

# re-run automatically whenever the file changes (recent Node.js)
node --watch hello.js

Running node with no file opens the REPL, an interactive prompt much like the browser console. Leave it with .exit or by pressing Ctrl+C twice.

Create an npm project

Real projects start with a package.json, which records the project name, its dependencies and the commands you run often. npm init -y creates one with default answers.

mkdir my-app
cd my-app
npm init -y
npm install --save-dev prettier

After adding a start script by hand, the file looks roughly like this (default fields vary a little between npm versions):

{
  "name": "my-app",
  "version": "1.0.0",
  "main": "index.js",
  "scripts": {
    "start": "node index.js"
  },
  "devDependencies": {
    "prettier": "^3.0.0"
  }
}

Installing packages creates a node_modules folder and a package-lock.json file. Keep node_modules out of Git by listing it in .gitignore, since it can always be rebuilt. Do commit package-lock.json: it pins exact versions so everyone installs the same thing.

Set up your editor

Visual Studio Code understands JavaScript out of the box, so autocomplete, go-to-definition and rename all work immediately. A few extensions make it better:

• ESLint flags likely bugs and style problems as you type.
• Prettier formats indentation, quotes and line breaks consistently.
• Error Lens (optional) shows diagnostics inline next to the offending line.

Add // @ts-check to the top of a JavaScript file and VS Code will type-check it with the TypeScript engine. Describe types in JSDoc comments and mistakes surface before you run anything.

// @ts-check

/**
 * @param {number} price
 * @param {number} rate
 */
function withTax(price, rate) {
  return price * (1 + rate);
}

withTax("1000", 0.1); // the editor underlines the string argument

Summary

• The browser console is the fastest way to experiment, with nothing to install.
• With Node.js installed, node file.js runs a script and npm manages packages.
• Projects begin with npm init -y; ignore node_modules, commit package-lock.json.
• VS Code plus ESLint, Prettier and // @ts-check catches many beginner mistakes early.