JS Injector

Run custom JavaScript before page load and block or replace specific site JavaScript files.

JS Injector is a powerful, lightweight developer extension designed to inject custom JavaScript before page load and intercept, block, or redirect external website scripts at the network layer. Built for web developers, reverse engineers, security researchers, and power users, JS Injector gives you complete control over website execution without requiring third-party userscript managers. KEY FEATURES 1. Document-Start Execution (MAIN World) Inject and execute your custom JavaScript code at document_start—before the page constructs the DOM or runs inline scripts. Runs directly in the page's real execution context (MAIN world) with full access to window, prototypes, and native APIs. 2. Network Script Blocking & Redirection (Declarative Net Request) Intercept external scripts at the browser network level using Chrome's native Declarative Net Request engine: - Block tracking, analytics, and third-party scripts. - Redirect scripts to custom online URLs (e.g. target.js -> https://your-server.com/custom.js). - Redirect scripts to local development servers (e.g. target.js -> http://localhost:3000/custom.js). 3. Built-In Monaco Code Editor Enjoy a desktop-class code editing experience directly in your browser: - Full syntax highlighting for JavaScript and TypeScript. - Dark and light theme support. - Keyboard shortcuts (Ctrl+S to save). - Real-time syntax validation. 4. Comprehensive Predefined Rules Library Get started instantly with one-click built-in rule templates: - Mock API Response (Fetch & XHR): Return mock JSON payloads with custom status codes and headers. - Network Request Logger: Inspect full URLs, request payloads, headers, response status, and response bodies in the console. - Redirect Script to Online URL: Swap external bundles with hosted files via Declarative Net Request. - Replace Whole Script or ES Module: In-memory module remapping with automatic relative import resolution. - Image URL Override: Replace image assets across img src, srcset, picture elements, and background styles. - Block Analytics & Telemetry: Block common tracking scripts and stub window globals. - No-Script & Media Blocker: Lightweight browsing modes. 5. Domain-Specific Targeting Target specific domains, subdomains, or all websites using standard Chrome match patterns (e.g. *://*.example.com/* or https://yoursite.com/*). 6. 100% Private and Local All rules, scripts, and configurations are stored entirely inside your browser's local storage. JS Injector collects zero telemetry, requires no accounts, and makes no external network requests. PERMISSIONS EXPLAINED - Declarative Net Request: Used to block or redirect specific script requests matching your configured rules. - User Scripts: Used to register and execute your custom JavaScript code at document_start. - Storage: Used to save your custom rules locally. - Host Permissions (<all_urls>): Required so your configured rules can execute on the target websites you specify.