compose-reducers.js

export default function composeReducers(reducers) {
  return (state, action) => {
    let reducerCount = reducers.length;
    let sideEffects = [];
    let noUpdateCount = 0;

    const reducedResult = reducers.reduceRight((prevState, reducer, index) => {
      // This is to handle the asymmetry in the useReducerWithSideFffect API.
      // Whereas regular reducers have a symmetric API that is state in, state out
      // useReducerWithSideEffects has state in, state+sideEffects out
      let state;
      if (index === reducerCount - 1) {
        state = prevState;
      } else {
        state = prevState.newState;
      }

      const result = reducer(state, action);

      const isNoUpdate = typeof result === 'symbol';

      let returnValue;

      if (isNoUpdate) {
        noUpdateCount++;

        // If this is the last reducer and we have no update, then we must
        // return the return value of the reducer, which is a Symbol.
        // That way, we can return it from the entire composed reducer.
        if (index === 0 && noUpdateCount === reducerCount) {
          returnValue = result;
        }
        // Otherwise, we must NOT return the Symbol, or else this loop will crash.
        else {
          returnValue = {
            newState: state,
          };
        }
      }
      // If the return value is not a symbol, then we always just return whatever
      // the reducer gave us.
      else {
        returnValue = result;
      }

      if (result && Array.isArray(result.newSideEffect)) {
        sideEffects = sideEffects.concat(result.newSideEffect);
      }

      return returnValue;
    }, state);

    const noUpdateOccurred = noUpdateCount === reducerCount;

    if (noUpdateOccurred) {
      return reducedResult;
    }

    return {
      newState: reducedResult && reducedResult.newState,
      newSideEffect: sideEffects,
    };
  };
}

if i have n reducers and the final one returns no update would my expecation be to get the updates from the first say 8/9 reducers? I would think so?

Yup! It behaves just like Redux’s composeReducers in that (and every other) regard.

ah .yeah, its been so loong since ive reeduxed makes sense, the gist looks good? would definitely ttake pr

Nice. I’ll whip somethin’ up