On the Expressive Power of Hook-Indexed State: Comprehending State in the React UI Library (Extended Version)

Toby Ueno, Ezra Cooper, Simon Fowler, Sam Lindley

React is currently the most popular JavaScript library for front-end web development. It endows UI components with state via the useState construct, through a mechanism that we name hook-indexed state. However, React's implementation of hook-indexed state is both fragile and poorly understood. Programmers frequently experience unexpected behavior, and the benefits of programming in this paradigm are wholly unclear. We define $λ^\mathsf{react}_\mathsf{hook}$, a core calculus that captures the essence of hook-indexed state, and give it a type system to statically rule out unexpected behavior. We then translate $λ^\mathsf{react}_\mathsf{hook}$ into a calculus $λ^\mathsf{react}_\mathsf{ref}$ based on ML-style references, and prove that this translation preserves typability and semantics, thus showing that hook-indexed state is expressible in terms of standard references. The translation is local and admits a compositional definition, but involves significant code duplication. As well as illuminating the essence of state in React, our observations can inform the design of future reactive programming systems.