What is fbjs?

FBJS is a shared toolkit of utility functions that Facebook uses across its own JavaScript projects. Think of it as an internal library, like a collection of helper tools that the company's teams reuse rather than rewriting the same code over and over. The main benefit is that Facebook can publish code once and have multiple projects (like React and Relay) pull from the same source, making it easier to ship and maintain those larger libraries. The way it works is straightforward: utility modules are stored in a central place, then converted into a format that other projects can import and use. When another Facebook project needs one of these utilities, a build tool rewrites the import statements automatically so they point to the shared fbjs package instead of local copies. For example, if a project tries to require a function called emptyFunction, the tool intercepts that and converts it to import from fbjs/lib/emptyFunction instead. This keeps everything in sync without duplicating code. The README is honest about scope: this library is built for Facebook's internal use and isn't really designed to be a general-purpose open source tool for everyone else. The team explicitly notes that APIs might change without warning and they probably won't add features just because external users request them, they're optimizing for Facebook's own needs first. Anyone using it from outside the company should expect things to shift around. The project is still evolving. The README lists some open questions about adding type annotations and eventually splitting it into smaller, more specialized packages rather than shipping one big bundle. For now, it's a pragmatic solution to the problem of code sharing across Facebook's JavaScript ecosystem, letting teams move fast without worrying about where shared code lives or how to keep multiple copies in sync.