Tampermonkey Userscript Development

Expert guidance for writing Tampermonkey userscripts - browser scripts that modify web pages, automate tasks, and enhance browsing experience.

Quick Start Template

// ==UserScript==
// @name         My Script Name                    // <- CUSTOMISE: Unique script name
// @namespace    https://example.com/scripts/      // <- CUSTOMISE: Your unique namespace
// @version      1.0.0                             // <- INCREMENT on updates
// @description  Brief description of the script   // <- CUSTOMISE: What it does
// @author       Your Name                         // <- CUSTOMISE: Your name
// @match        https://example.com/*             // <- CUSTOMISE: Target URL pattern
// @grant        none                              // <- ADD permissions as needed
// @run-at       document-idle                     // <- ADJUST timing if needed
// ==/UserScript==

(function() {
    'use strict';

    // Your code here
    console.log('Script loaded!');
})();

Essential Header Tags

For complete header documentation, see: header-reference.md

URL Matching Quick Reference

// Exact domain                  // @match https://example.com/*
// All subdomains                // @match https://*.example.com/*
// HTTP and HTTPS                // @match *://example.com/*
// Exclude paths (with @match)   // @exclude https://example.com/admin/*

For advanced patterns (regex, @include, specific paths), see: url-matching.md

@grant Permissions Quick Reference

// Disable sandbox (no GM_* except GM_info)
// @grant none

// Cross-origin requests require @connect
// @grant GM_xmlhttpRequest
// @connect api.example.com
// @connect *.googleapis.com

For complete permissions guide, see: header-reference.md

@run-at Injection Timing

Common Patterns

These patterns are used frequently. Brief summaries are below - load patterns.md for full implementations with code examples.

• Wait for Element - Promise-based MutationObserver that resolves when a CSS selector appears in the DOM, with configurable timeout
• SPA URL Change Detection - Detect navigation in single-page apps using window.onurlchange grant or History API interception
• Cross-Origin Request - Fetch data from external APIs using GM_xmlhttpRequest with @connect domain whitelisting. See also http-requests.md
• Add Custom Styles - Inject CSS with GM_addStyle to restyle pages or hide elements. See also api-dom-ui.md
• Persistent Settings - Store user preferences with GM_setValue/GM_getValue and expose toggle via GM_registerMenuCommand. See also api-storage.md
• DOM Mutation Observation - Watch for dynamically added content with MutationObserver (debounced variant included)
• Element Manipulation - Inject HTML, remove/hide elements, replace text across the page
• Keyboard Shortcuts - Simple handlers and a shortcut manager with modifier key support
• Data Extraction - Extract table data to arrays/objects, collect and filter page links
• Error Handling - Safe wrapper for try/catch and async retry with exponential backoff

External Resources

// @require - Load external scripts
// @require https://code.jquery.com/jquery-3.6.0.min.js#sha256-/xUj+3OJU...
// @require tampermonkey://vendor/jquery.js         // Built-in library

// @resource - Preload and inject external CSS
// @resource myCSS https://example.com/style.css    // Then: GM_addStyle(GM_getResourceText('myCSS'))
// @grant GM_getResourceText
// @grant GM_addStyle

What Tampermonkey Cannot Do

Userscripts have limitations:

• Access local files - Cannot read/write files on your computer
• Run before page scripts - In isolated sandbox mode, page scripts run first
• Access cross-origin iframes - Browser security prevents this
• Persist across machines - GM storage is local to each browser
• Bypass all CSP - Some very strict CSP cannot be bypassed

Most limitations have workarounds - see common-pitfalls.md.

When Generating Userscripts

Always include in your response:

1. Explanation - What the script does (1-2 sentences)
2. Complete userscript - Full code with all headers in a code block
3. Installation - "Copy/paste into Tampermonkey dashboard" or "Save as .user.js"
4. Customisation points - What the user can safely modify (selectors, timeouts, etc.)
5. Permissions used - Which @grants and why they're needed
6. Browser support - If Chrome-only, Firefox-only, or universal

Pre-Delivery Checklist

Before returning a userscript, verify:

Critical (Must Pass)

• No hardcoded API keys, tokens, or passwords
• @match is specific (not *://*/*)
• All external URLs use HTTPS
• User input sanitised before DOM insertion

Important (Should Pass)

• Wrapped in IIFE with 'use strict'
• All @grant statements are necessary
• @connect includes all external domains
• Error handling for async operations
• Null checks before DOM manipulation

Recommended

• @version follows semantic versioning (X.Y.Z)
• Works in both Chrome and Firefox
• Comments explain non-obvious code

For complete security checklist, see: security-checklist.md

Reference Files Guide

Load these on-demand based on user needs: