fbjs

FBJS is an internal toolkit of JavaScript utilities that Facebook uses across its own projects. Think of it as a shared toolbox — instead of writing the same helper functions over and over in React, Relay, and other Facebook libraries, this collection lets teams reuse code and avoid duplication.

At its core, the project solves a simple problem: when you have many related projects built by the same organization, you end up writing similar utility code in each one. FBJS acts as a central place to put those shared pieces, so different teams can pull from the same source rather than each maintaining their own version. It's similar to how a large company might have an internal library of design patterns or common tools that all teams can access.

The project was built with Facebook's internal workflow in mind, using a system called @providesModule that makes it easier to manage dependencies across their codebase. It's published publicly so the code is available, but the README makes clear that this is primarily a Facebook-facing tool — the maintainers are explicit that they optimize for their own use cases first and don't plan to support external developers in the same way they would for a public-facing library like React. This means APIs can change without warning, and feature requests from outside the company are unlikely to be prioritized.

If you work at Facebook or maintain a Facebook project (like an open-source framework), you might use FBJS to avoid reinventing utility functions. If you're an external developer, the code is there to learn from or use, but you should be aware that breaking changes might happen without much notice. The library is licensed under MIT, so technically anyone can use it — just with the caveat that it wasn't designed with external stability in mind.