Roact is a declarative Lua UI library similar to Facebook's React. It exposes a very similar API and implements nearly identical semantics.
This documentation is based on the structure of React's documentation, but is a work in progress. Many things from React also apply to Roact, but if you find anything missing or incorrect, open an issue on GitHub!
This documentation assumes some familiarity with Lua. If you're new to Lua, Programming in Lua by Roberto Ierusalimschy is a good introduction, and the first edition (for Lua 5.0) is available online for free.