[2] Its purpose is to block nonfree nontrivial JavaScript programs and allow free or trivial JS in a user's web browser.

The add-on was written to address the so-called "JavaScript Trap"[3] first described by Richard Stallman in 2009,[4] a situation in which many users unknowingly run proprietary software in their web browsers.

GNU LibreJS refuses to run nonfree programs that are present in many websites.

[7][8] The add-on allows site whitelisting and has an email address detection system for users to contact page webmasters in order to persuade them into making their JavaScript code compliant with LibreJS.

[9] According to the Free Software Foundation, many websites (if not whitelisted) break while using the add-on, due to the pervasiveness of proprietary JavaScript code on the web.