Redux : Manually implementing Combine Reducer

manual_combine_reducers.js

// Reference : https://egghead.io/lessons/javascript-redux-implementing-combinereducers-from-scratch

const combineReducers = (reducers) => {
  return ( state = {}, action) => {
      const keys = Object.keys(reducers); 
      const arr = keys.reduce((accumulator, key) => {
        // since accumulator is created inside reducer function we don't have to worry about if it's mutable or not.
        // reducer still stays as a pure function.
        accumulator[key] = reducers[key](state[key],action);
        return accumulator;
      }, {});
    return arr;
  };
}

// functions can be combined 
const combineReducers = (reducers) => {
  return ( state = {}, action) => {
      return Object.keys(reducers).reduce((accumulator, key) => 
        accumulator[key] = reducers[key](state[key],action);
        return accumulator;
      }, {});
  };
}