statelyai/xstate

XState is a library for managing complex application logic using state machines and statecharts. A state machine is a way of modeling behavior as a fixed set of states (like "logged out", "loading", "logged in") with defined rules for how events cause transitions between them. Statecharts extend this idea with nesting, parallel states, and history, making it practical for real-world applications. The problem XState solves is that as applications grow, the logic controlling what happens when (who can click what button, what network call to make, what to show on screen) becomes tangled and hard to predict. State machines impose structure: because every state and every transition is explicit, unexpected combinations of events are handled predictably rather than silently causing bugs. XState uses an actor model, each machine instance runs as an independent actor that sends and receives messages, making it straightforward to coordinate multiple concurrent processes. It works in both JavaScript and TypeScript and has no external dependencies. A companion package called @xstate/store handles simpler cases where you just need a small shared store without the full state machine system. There is also a visual editor called Stately Studio where you can draw state machines as diagrams and export the code. Someone would use XState when building applications with complex, multi-step workflows, forms, checkout flows, authentication sequences, or multi-stage data-fetching, where unstructured if/else logic becomes difficult to maintain.