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/*

// Specific path
// @match https://example.com/app/*

// Exclude paths (use with @match)
// @exclude https://example.com/admin/*

For advanced patterns (regex, @include), 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

Wait for Element

function waitForElement(selector, timeout = 10000) {
    return new Promise((resolve, reject) => {
        const element = document.querySelector(selector);
        if (element) return resolve(element);

        const observer = new MutationObserver((mutations, obs) => {
            const el = document.querySelector(selector);
            if (el) {
                obs.disconnect();
                resolve(el);
            }
        });

        observer.observe(document.body, { childList: true, subtree: true });

        setTimeout(() => {
            observer.disconnect();
            reject(new Error(`Element ${selector} not found`));
        }, timeout);
    });
}

// Usage
waitForElement('#my-element').then(el => {
    console.log('Found:', el);
});

SPA URL Change Detection

// @grant window.onurlchange

if (window.onurlchange === null) {
    window.addEventListener('urlchange', (info) => {
        console.log('URL changed to:', info.url);
        // Re-run your modifications
    });
}

Cross-Origin Request

// @grant GM_xmlhttpRequest
// @connect api.example.com

GM_xmlhttpRequest({
    method: 'GET',
    url: 'https://api.example.com/data',
    headers: { 'Content-Type': 'application/json' },
    onload: function(response) {
        const data = JSON.parse(response.responseText);
        console.log(data);
    },
    onerror: function(error) {
        console.error('Request failed:', error);
    }
});

Add Custom Styles

// @grant GM_addStyle

GM_addStyle(`
    .my-custom-class {
        background: #f0f0f0;
        padding: 10px;
        border-radius: 5px;
    }

    #hide-this-element {
        display: none !important;
    }
`);

Persistent Settings

// @grant GM_setValue
// @grant GM_getValue
// @grant GM_registerMenuCommand

const settings = {
    enabled: GM_getValue('enabled', true),
    theme: GM_getValue('theme', 'dark')
};

GM_registerMenuCommand('Toggle Enabled', () => {
    settings.enabled = !settings.enabled;
    GM_setValue('enabled', settings.enabled);
    location.reload();
});

For more patterns, see: patterns.md

External Resources

@require – Load External Scripts

// @require https://code.jquery.com/jquery-3.6.0.min.js

// With integrity hash (recommended)
// @require https://code.jquery.com/jquery-3.6.0.min.js#sha256-/xUj+3OJU...

// Built-in libraries
// @require tampermonkey://vendor/jquery.js

@resource – Preload Resources

// @resource myCSS https://example.com/style.css
// @grant GM_getResourceText
// @grant GM_addStyle

const css = GM_getResourceText('myCSS');
GM_addStyle(css);

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

Example response format:

This script adds a dark mode toggle to Example.com and remembers the user’s preference.

// ==UserScript==
// @name         Example.com Dark Mode
// ...
// ==/UserScript==

Installation: Copy/paste into Tampermonkey dashboard Customisation: Change DARK_BG_COLOR to adjust the background colour Permissions: Uses GM_setValue/GM_getValue to remember preference Browsers: Chrome and Firefox

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:

Core Documentation

API Reference

Troubleshooting & Quality