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Hello, World in JavaScript

Start quickly in the browser or with Node.js. JavaScript runs top‑to‑bottom; no special main function is required.

Browser (script tag)

Include a script and log to the console. Scripts in <body> execute when encountered; placing them at the end avoids blocking rendering.

<!doctype html>
<html>
  <body>
    <h1>Hello</h1>
    <script>
      console.log("Hello, world!")
      // alert("Hello, world!") // modal popup (optional)
    </script>
  </body>
</html>

Open DevTools (Console) to see logs.

Browser modules (ESM)

Use <script type="module"> for modern module features. Module scripts are deferred by default and have their own scope.

<script type="module">
  import { sum } from "./math.js"
  console.log(sum(2, 3))
</script>

Node.js (CLI)

Create a file and run it with Node.

// hello.js
console.log("Hello, world!")

Run: node hello.js

ES modules vs CommonJS

  • ESM (recommended): set "type": "module" in package.json or use .mjs
  • CJS (legacy): require()/module.exports Top‑level await is supported in ESM in modern Node and browsers.

Console API and formatting

console.log("x=%d", 42)
console.info("info")
console.warn("warn")
console.error("error")
console.table([{ a: 1 }, { a: 2 }])

Template literals support interpolation.

const name = "JS"
console.log(`Hello, ${name}`)

Strict mode (safety)

Enable stricter semantics; prevents accidental globals and throws on some silent errors. ESM is strict by default.

'use strict'

Summary

  • Use <script> or Node to run code
  • Prefer ESM (type: module); use DevTools console for quick tests
  • console.* for logging; template literals for formatting
 
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