React useReducer: How to combine multiple reducers?

Question

I'm not a Javascript expert so I wondered if anyone has an "elegant" way to combine multiple reducers to create a global state(Like Redux). A function that does not affect performance when a state updating multiple components etc..

Let's say I have a store.js

import React, { createContext, useReducer } from "react";
import Rootreducer from "./Rootreducer"

export const StoreContext = createContext();

const initialState = {
    ....
};

export const StoreProvider = props => {
  const [state, dispatch] = useReducer(Rootreducer, initialState);

  return (
    <StoreContext.Provider value={[state, dispatch]}>
      {props.children}
    <StoreContext.Provider>
  );
};

Rootreducer.js

import Reducer1 from "./Reducer1"
import Reducer2 from "./Reducer2"
import Reducer3 from "./Reducer3"
import Reducer4 from "./Reducer4"

const rootReducer = combineReducers({
Reducer1,
Reducer2,
Reducer3,
Reducer4
})

export default rootReducer;

Answer 1

Combine slice reducers (combineReducers)

The most common approach is to let each reducer manage its own property ("slice") of the state:

const combineReducers = (slices) => (state, action) =>
  Object.keys(slices).reduce( // use for..in loop, if you prefer it
    (acc, prop) => ({
      ...acc,
      [prop]: slices[prop](acc[prop], action),
    }),
    state
  );

Example:

import a from "./Reducer1";
import b from "./Reducer2";

const initialState = { a: {}, b: {} }; // some state for props a, b
const rootReducer = combineReducers({ a, b });

const StoreProvider = ({ children }) => {
  const [state, dispatch] = useReducer(rootReducer, initialState);
  // Important(!): memoize array value. Else all context consumers update on *every* render
  const store = React.useMemo(() => [state, dispatch], [state]);
  return (
    <StoreContext.Provider value={store}> {children} </StoreContext.Provider>
  );
};

Combine reducers in sequence

Apply multiple reducers in sequence on state with arbitrary shape, akin to reduce-reducers:

const reduceReducers = (...reducers) => (state, action) =>
  reducers.reduce((acc, nextReducer) => nextReducer(acc, action), state);

Example:

const rootReducer2 = reduceReducers(a, b);
// rest like in first variant

Combine multiple useReducer Hooks

You could also combine dispatch and/or state from multiple useReducers, like:

const combineDispatch = (...dispatches) => (action) =>
  dispatches.forEach((dispatch) => dispatch(action));

Example:

const [s1, d1] = useReducer(a, {}); // some init state {} 
const [s2, d2] = useReducer(b, {}); // some init state {} 

// don't forget to memoize again
const combinedDispatch = React.useCallback(combineDispatch(d1, d2), [d1, d2]);
const combinedState = React.useMemo(() => ({ s1, s2, }), [s1, s2]);

// This example uses separate dispatch and state contexts for better render performance
<DispatchContext.Provider value={combinedDispatch}>
  <StateContext.Provider value={combinedState}> {children} </StateContext.Provider>
</DispatchContext.Provider>;

In summary

Above are the most common variants. There are also libraries like use-combined-reducers for these cases. Last, take a look at following sample combining both combineReducers and reduceReducers:

const StoreContext = React.createContext();
const initialState = { a: 1, b: 1 };

// omit distinct action types for brevity
const plusOneReducer = (state, _action) => state + 1;
const timesTwoReducer = (state, _action) => state * 2;
const rootReducer = combineReducers({
  a: reduceReducers(plusOneReducer, plusOneReducer), // aNew = aOld + 1 + 1
  b: reduceReducers(timesTwoReducer, plusOneReducer) // bNew = bOld * 2 + 1
});

const StoreProvider = ({ children }) => {
  const [state, dispatch] = React.useReducer(rootReducer, initialState);
  const store = React.useMemo(() => [state, dispatch], [state]);
  return (
    <StoreContext.Provider value={store}> {children} </StoreContext.Provider>
  );
};

const Comp = () => {
  const [globalState, globalDispatch] = React.useContext(StoreContext);
  return (
    <div>
      <p>
        a: {globalState.a}, b: {globalState.b}
      </p>
      <button onClick={globalDispatch}>Click me</button>
    </div>
  );
};

const App = () => <StoreProvider> <Comp /> </StoreProvider>
ReactDOM.render(<App />, document.getElementById("root"));

//
// helpers
//

function combineReducers(slices) {
  return (state, action) =>
    Object.keys(slices).reduce(
      (acc, prop) => ({
        ...acc,
        [prop]: slices[prop](acc[prop], action)
      }),
      state
    )
}

function reduceReducers(...reducers){ 
  return (state, action) =>
    reducers.reduce((acc, nextReducer) => nextReducer(acc, action), state)
}

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