mobxjs/mobx

MobX is a library for managing application state, the in-memory data that drives what your app shows on screen. Its core idea is captured in one line from the README: anything that can be derived from the application state should be derived automatically. You declare some pieces of data as observable, then write the rest of your app, UI components, computed values, side effects, as if it were plain code that simply reads from that data. MobX watches what each piece of code actually touches and, when the underlying data changes, re-runs exactly the parts that depended on it. In practice this means you mutate state with normal JavaScript assignments instead of dispatching actions through reducers or immutable copies. The example in the README wraps a small object containing a counter and an increase method in makeAutoObservable, then renders it with a React component wrapped in the observer helper from mobx-react-lite, updating the counter via plain assignment causes only the components that read it to re-render. The library calls itself signal-based and built on functional reactive programming, and it builds a dependency graph at runtime so you do not have to hand-tune memoization or selectors. You would reach for it when your app's state has grown complex enough that prop-drilling or ad-hoc updates feel painful, and you want something less ceremonial than a strict reducer-based store. MobX is unopinionated about which UI framework you use, it works with React but also lets you keep state logic outside of any UI framework, which makes that logic portable and easier to test. The codebase is written in TypeScript.