JavaScript in Different Environments

JavaScript was born in the browser, but today it runs in many environments.

Browser:

• Manipulates the DOM (HTML elements).
• Handles user events (clicks, form submissions).
• Communicates with servers via fetch or XMLHttpRequest.
• Uses browser APIs like localStorage, geolocation, etc.

Example – changing page content:

<button id="btn">Click me</button>
<script>
  document.getElementById("btn").addEventListener("click", () => {
    document.body.innerHTML += "<p>Button clicked!</p>";
  });
</script>

Node.js: Runs on servers, desktops, or embedded devices. Provides file system access, networking, and operating system features. Uses the require (or import) syntax for modules.

Example – reading a file:

const fs = require("fs");
fs.readFile("file.txt", "utf8", (err, data) => {
  if (err) throw err;
  console.log(data);
});

Other environments:

• Deno: a secure runtime for TypeScript and JavaScript.

• Bun: a fast all‑in‑one JavaScript runtime.

• Mobile apps: React Native, NativeScript.

• Desktop apps: Electron, NW.js.

Each environment provides its own set of APIs, but the core language remains the same.