JavaScript Execution: Browser vs Node.js

JavaScript can run in two major environments: inside a web browser and on the server using Node.js. Both environments execute JavaScript, but they provide different tools, capabilities, and use cases.

JavaScript in the Browser

Every modern browser (like Chrome, Firefox, Safari, or Edge) has a built-in JavaScript engine. These engines are responsible for reading and executing JavaScript code directly on the user’s device.

Examples of JavaScript Engines:

Where the Code Runs:

JavaScript in the browser runs inside a web page. It can interact with:

• HTML elements
• CSS styles
• User events (like clicks and keystrokes)

<button onclick="sayHello()">Click Me</button>

<script>
  function sayHello() {
    alert("Hello from the browser!");
  }
</script>

Browser Environment Features:

• Access to the DOM (Document Object Model)
• Access to Window, Document, Navigator, and other browser APIs
• Limited access to the file system (usually none)

JavaScript in Node.js

Node.js is a JavaScript runtime built on Chrome’s V8 engine that allows JavaScript to run outside of the browser, mainly on servers.

With Node.js, you can build:

• Web servers
• APIs
• File system tools
• Backend applications

Where the Code Runs:

In a terminal or command line, using .js files.

Example:

// save as app.js
console.log("Hello from Node.js!");

Run it with:

node app.js

Node.js Environment Features:

• Access to the file system, network, and OS
• No access to browser-specific objects like window or document
• Ability to use modules (like fs, http, path)
• Event-driven architecture (similar to browser, but with different APIs)

Key Differences: Browser vs Node.js

Summary

JavaScript runs in both the browser and on the server, but the environments are very different. Browsers give access to the DOM and user interface, while Node.js allows JavaScript to work on the backend, interact with files, and handle server operations.