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5.2.0 • Public • Published


FunState is a React architecture and library for doing fractal, compositional state in a way that is type-safe, testable, and easy to refactor.

Getting Started

FunState works with any react 16.8+ application. Usage without TypeScript works but isn't recommended.

  1. npm install -S fun-state
  2. Pick or create a component hold the FunState:
import {index} from 'accessors-ts';
import {useFunState} from 'fun-state';
import {Todo, TodoItem} from './Todo/TodoItem';

// Define an interface for your App's state
interface TodoApp {
  users: Todo[]

// Define an initial state:
const initialAppState: TodoApp = {
  todos: [],

// Create a FunState instance within a React.FunctionalComponent (uses react hooks)
const App = () => {
  const state = useFunState(initialAppState);
  const {todos} = funState.get();
  return (
    {/* Child components can get the root state directly */}
    <SelectAll state={state} />
    {, i) => (
      {/* or focus down to the state the component needs to interact with */}
      <TodoItem state={state.prop('todos').focus(index(i))} />
  1. Create child components focused on a piece of your state:
// MyChildComponent.tsx
// Should be imported into the parent state interface
export type ChildState = boolean;

export const MyChildComponent: React.FC<{state: FunState<ChildState>}> = ({state}) => (
  <input type="checkbox" checked={state.get()} onChange=(e => state.set(e.currentTarget.checked))>

More examples

See fun-state-examples for a sample standalone application.

When to useFunState

  • When you're in a situation where you would gain benefit from redux or other state-managment libraries.
  • You want composable/modular state
  • You want to gradually try out another state management system without fully converting your app.

When not to useFunState

  • When your data or component heirachy is mostly flat.
  • When our app is not as complex as the token TodoApp then adding these concepts and tools is probably overkill.
  • You're avoiding FunctionComponents


  • Keep your FunState Apps simple and delegate the complex logic to pure child components, using .sub() where practical.
  • Use Accessor composition to drill down into deep parts of your tree or operate on multiple items. See ./TodoApp or accessor-ts docs for examples.
  • If child components need data from multiple places in the state tree, you can create and pass more than one FunState or just pass the root and then query out what you need with Accessors.
  • Unit test your updaters and snapshot test your components.
  • As usual, memoizing event handlers can help if you run into rendering performance problems.



<State>(initialState: State) => FunState<State>

Creates an react-hooks based FunState instance with a starting state.


<State>(initialState: State) => FunState<State>

Creates a library-agnostic instance of the state machine with a starting state. This is useful when unit testing functions or components that take a FunState instance.


<State>({getState, modState}: StateEngine<State>): FunState<State>

Creates an instance of funState given a custom StateEngine. If you want to add support for preact or other libraries with things like hooks you want this.


See accessor-ts


export interface FunState<State> {
  get: () => State
  /** Query the state using an accessor */
  query: <A>(acc: Accessor<State, A>) => A[]
  /** Transform the state with the passed function */
  mod: Updater<State>
  /** Replace the state */
  set: (val: State) => void
  /** Create a new FunState focused at the passed accessor */
  focus: <SubState>(acc: Accessor<State, SubState>) => FunState<SubState>
  /** focus state at passed key (sugar over `focus(prop(k))`) */
  prop: <K extends keyof State>(key: K) => FunState<State[K]>

Data structure that holds the state along with a stateful function that updates it.


<State>(fs: FunState<State>) => (part: Partial<State>) => void

Mutably merge a partial state into a FunState

TODO / Contributing

  • Give feedback!
  • Add performance benchmarks
  • File bugs
  • Improve documentation
  • Add more examples


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  • jethrolarson