quick react context with reducer

createCtx.tsx

export function createCtx<StateType, ActionType>(
  reducer: React.Reducer<StateType, ActionType>,
  initialState: StateType,
) {
  const defaultDispatch: React.Dispatch<ActionType> = () => initialState // we never actually use this
  const ctx = React.createContext({
    state: initialState,
    dispatch: defaultDispatch, // just to mock out the dispatch type and make it not optioanl
  })
  function Provider(props: React.PropsWithChildren<{}>) {
    const [state, dispatch] = React.useReducer<React.Reducer<StateType, ActionType>>(reducer, initialState)
    return <ctx.Provider value={{ state, dispatch }} {...props} />
  }
  return [ctx, Provider] as const
}





// ------------------------------------------------------------------------------
// save the above as a separate file (createCtx.tsx), and then use as below in another file
// ------------------------------------------------------------------------------
// usage
const initialState = { count: 0 }
type AppState = typeof initialState
type Action =
  | { type: 'increment' }
  | { type: 'add'; payload: number }
  | { type: 'minus'; payload: number }
  | { type: 'decrement' }

function reducer(state: AppState, action: Action): AppState {
  switch (action.type) {
    case 'increment':
      return { count: state.count + 1 }
    case 'decrement':
      return { count: state.count - 1 }
    case 'add':
      return { count: state.count + action.payload }
    case 'minus':
      return { count: state.count - action.payload }
    default:
      throw new Error()
  }
}
const [ctx, CountProvider] = createCtx(reducer, initialState)
export const CountContext = ctx

// top level example usage
export function App() {
  return (
    <CountProvider>
      <Counter />
    </CountProvider>
  )
}

// example usage inside a component
function Counter() {
  const { state, dispatch } = React.useContext(CountContext)
  return (
    <div>
      Count: {state.count}
      <button onClick={() => dispatch({ type: 'increment' })}>+</button>
      <button onClick={() => dispatch({ type: 'add', payload: 5 })}>+5</button>
      <button onClick={() => dispatch({ type: 'decrement' })}>-</button>
      <button onClick={() => dispatch({ type: 'minus', payload: 5 })}>+5</button>
    </div>
  )
}
  
thanks to https://gist.github.com/sw-yx/f18fe6dd4c43fddb3a4971e80114a052 for this gist