JavaScript Restrictor

Disclaimer: This is a research project under development, see the issue page and the webextension home page for more details about the current status.

A JS-enabled web page can access any of the APIs that a web browser provides. The user has only a limited control and some APIs cannot be restricted by the user easily. JavaScript Restrictor aims to improve the user control of the web browser. Similarly to a firewall that controls the network traffic, JavaScript Restrictor controls the APIs provided by the browser. The goal is to improve the privacy and security of the user running the extension.


JavaScript Restrictor (JSR) is a browser extension with support for multiple browsers: Firefox, Google Chrome, and Opera. The extension also works with Brave, Microsoft Edge, and most likely any Chromium-based browser. Let us know if you want to add the extension to additional stores.


Various websites collect information about users without their awareness. The collected information is used to track users. Malicious websites can fingerprint user browsers or computers. JavaScript Restrictor protects the user by restricting or modifying several web browser APIs used to create side-channels and identify the user, the browser or the computer. JavaScript Restrictor can block access to JavaScript objects, functions and properties or provide a less precise implementation of their functionality, for example, by modifying or spoofing values returned by the JS calls. The goal is to mislead websites by providing false data or no data at all.

Another goal of the extension is not to break the visited websites. As the deployment of JavaScript only websites rise, it is necessary to fine-tune the API available to the websites to prevent unsolicited tracking and protect against data thefts.

Protected APIs

JavaScript Restrictor currently supports modifying and restricting the following APIs (for more details visit levels of protection page):

Note that the spoofing and rounding actions performed by the extension can break the functionality of a website (e.g. Netflix). Please report to us any malfunction websites that do not track users.

Levels of Protection

JavaScript Restrictor provides four in-built levels of protection:

For more accurate description of the restrictions see levels of protection page.

The default level of protection can be set by a popup (clicking on JSR icon) or through options of the extension. Specific level of protection for specific domains can be set in options by adding them to the list of websites with specific level of protection. This can be done also by a popup during a visit of the website.


If you have any questions or you have spotted a bug, please let us know.

If you would like to give us feedback, we would really appreciate it.

Once you install the extension, see the test page for the working demo on how the extension can help in restricting JS capabilities.

License Information

This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.