With Arrays

If you'd prefer to stick with arrays, then you can write a reducer that just tackles single post objects.

export default function reducePost(post, action) {  if(post.id !== action.id) return post;  switch(action.type) {  case REQUEST_POST_BODY:    return Object.assign({}, post, { isFetching: true });  case RECEIVE_POST_BODY:    return Object.assign({}, post, { isFetching: false, body: action.body });  default:    return post;}

Your root reducer would become:

export default function posts(state = initialState, action) {  return state.map(post => reducePost(post, action);}

We're just running our new reducer over each post in the list, to return an updated array of posts. In this case, the unique id will ensure that only one item will be changed.

With Objects

If each item has a unique string/number id, then you can flip your array around and use an object instead.

const initialState = {  items: {    3: {id:3, title: '1984', isFetching:false},    6: {id:6, title: 'Mouse', isFetching:false}  };}

Then you can simplify your reducer.

switch (action.type) {case REQUEST_POST_BODY:  let id = action.id;  return Object.assign({}, state, {    [id]: Object.assign({}, state[id], { isFetching: true })  });case RECEIVE_POST_BODY:  let id = action.id;  return Object.assign({}, state, {    [id]: Object.assign({}, state[id], {      isFetching: false,      body: action.body    })  });default:  return state;}

If you're happy to experiment with some ES7 syntax too, you can enable the Object spread operator with Babel and rewrite the calls to Object.assign.

switch (action.type) {case REQUEST_POST_BODY:  let id = action.id;  return {    ...state,    [id]: {...state[id], isFetching: true }  };case RECEIVE_POST_BODY:  let id = action.id;  return {    ...state,    [id]: {      ...state[id],      isFetching: false,      body: action.body    }  };default:  return state;}

If you're not so keen on using the spread syntax, then it's still possible to make Object.assign a bit more palatable.

function $set(...objects) {  return Object.assign({}, ...objects); }case RECEIVE_POST_BODY:  let id = action.id;  return $set(state, {    [id]: $set(state[id], {      isFetching: false,      body: action.body    })  });

If I understand correctly, you are having trouble getting the specific post you want.

First of all, Having your reducer also update the array and the object in it, makes it hard to read and maintain. I suggest you watch this short video explaining about reducer composition with arrays.You can simplify your code by using the technique described there.

In your case, you would a posts reducer and a post reducer, while posts reducer calls the post reducer.

As for finding the right object to work on, Dan Prince's suggestion makes it easier.Having an object map instead of an array would make it easier for you. Relevant code snippet from Dan's answer:

const initialState = {  items: {    3: {id:3, title: '1984', isFetching:false},    6: {id:6, title: 'Mouse', isFetching:false}  ];}

I pretty much implemented Object reducers by using Object.assign, which works, but as our project has grown and we have added a bunch of dependent components it has become very inefficient and renders are very slow.

If I'd know about immer I would have used that from the start.

Essentially you use immer as follows, where the example has a layers object that looks like this:

const initialState = {  layers: {   'layer1': { selected: false },   'layer2': { selected: true }  }}

Reducers.js (extract)

import produce from 'immer'import has from 'lodash/has'export const layers = (state = null, action) => {  switch (action.type) {    case ADD_LAYER:      // Using `immer.produce` only for consistency       // clearly the whole object changes in this case.      return produce(state, layers => {        // Take a copy of the prebuilt layer        var layer = Object.assign({}, action.layer)        // Add some defaults        if (!has(layer, 'selected')) {          layer.selected = false        }        layers[action.id] = layer      })    case SELECT_LAYER:      return produce(state, layers => {        layers[action.id].selected = true      })    case UNSELECT_LAYER:      return produce(state, layers => {        layers[action.id].selected = false      })    default:      return state  }}

Actions.js (extract)

export const addLayer = id => ({  type: ADD_LAYER,  id})export const selectLayer = id => ({  type: SELECT_LAYER,  id})export const unSelectLayer = id => ({  type: UNSELECT_LAYER,  id})

References:

https://github.com/immerjs/immer

https://redux.js.org/recipes/structuring-reducers/immutable-update-patterns