combineReducers for Immutable.js and Redux

index.js

// combineReducers.js
function combineReducers(reducers) {
  return function(state, action) {
    return Object.entries(reducers).reduce((state, [subtree, reducer]) => {
      return state.update(subtree, reducer(action))
    }, state)
  }
}

// reducers.js

// Given a state tree of the following shape
// fromJS({
//   user: {
//     username: 'Bill'
//   },
//   posts: {
//     1: { /* ..post 1 data.. */ },
//     2: { /* ..post 2 data.. */ }
//   }
// })


// Note that the function name is exactly the same as the state subtree that it handles
function user(action) {
  return function(state) {
    switch (action.type) {
      case SET_USERNAME:
        return state.set('username', action.username)
      default:
        return state
    }
  }
}

function posts(action) {
  return function(state) {
    switch (action.type) {
      case DELETE_POST:
        return state.delete(action.postId)
      default:
        return state
    }
  }
}

export default combineReducers({ user, posts })