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Injecting JavaScript into a Website (Chrome)

This document explains common ways to inject custom JavaScript into websites using Chrome, without modifying the site’s source code. Typical use cases include automation, UI tweaks, learning/debugging, and personal productivity scripts.

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1. User Script (Tampermonkey / UserJS)

What it is

A userscript runs automatically on matching websites after the page loads.
Tampermonkey is the most popular userscript manager for Chrome.

When to use

• Persistent scripts
• Run on specific domains or pages
• Need access to page context and DOM
• Want easy enable/disable

Steps

1. Install Tampermonkey from Chrome Web Store
2. Create a new script
3. Add metadata and JavaScript code

Example

// ==UserScript==
// @name         Example Injector
// @namespace    https://example.com
// @version      1.0
// @description  Inject custom JS into a site
// @match        https://example.com/*
// @grant        none
// ==/UserScript==

(function () {
  'use strict';
  console.log('Injected via Tampermonkey');
})();