implement Redux's combineReducers from scratch

combineReducers.js

// accept a array of reducers
const combineReducers = (reducers) => {
  // returns a huge reducer, looks like below gist
  return (state = {}, action) => {
    return Object.keys(reducers).reduce(
      (nextState, key) => {
        nextState[key] = reducers[key](
          state[key],
          action
        )
        return nextState
      },
      {}
    )
  }
}

huge_reducer_sample.js

// a huge reducer looks like this
const todoApp = (state = {}, action) => {
  // 'huge reducer also returns a huge state'
  return {
    todos: todosReducer(
      state.todos,
      action
    ),
    visibilityFilter: VisibilityFilter(
      state.visibilityFilter,
      action
    )
  }
}

check Dan's redux tutorial videos