.JS

JavaScript source code

Introduction

A .js file holds JavaScript, the programming language of web pages, as plain text. Browsers run it in pages, and runtimes such as Node.js, Deno and Bun run it on servers and in the terminal.

Two other extensions say which module system a file uses in Node.js: .mjs for an ECMAScript module (import and export) and .cjs for a CommonJS module (require and module.exports).

Software

Structure

A JavaScript file is plain text, normally UTF-8, with no header. A file is either a classic script, whose top-level names become globals, or a module, which has its own scope and shares names only through import and export. The text alone does not always say which: a web page decides with <script type="module">, and Node.js decides from the extension or the "type" field of the nearest package.json.

import { readFile } from 'node:fs/promises';

export async function wordCount(path) {
  const text = await readFile(path, 'utf8');
  return text.split(/\s+/).length;
}
A small JavaScript module with one import and one export.