如何正确处理双嵌套数组（ReactJS + Redux）中的状态？

Question

I am trying to build the following: a forum where one can create a post, which triggers the ADD_POST, and the post is created, and added to the 'posts' object array. The each 'post' object is initialized with a 'comments' array that will hold comment texts ('commentTxt') entered inside that post.

let postReducer = function(posts = [],  action) {
  switch (action.type) {
    case 'ADD_POST':
      return [{
        id: getId(posts), //just calls a function that provides an id that increments by 1 starting from 0
        comments: [
          {
            id: getId(posts),
            commentTxt: ''
          }
        ]
      }, ...posts]

Then when the user enters that post, there is a comment section where the user can enter a comment text and a new object would be added (via 'ADD_COMMENT') to the 'posts.comments' array

  case 'ADD_COMMENT':
      return posts.map(function(post){
    //find the right 'post' object in the 'posts' array to update the correct 'comments' array. 
        if(post.id === action.id){
    //update 'comments' object array of a 'post' object by adding a new object that contains 'commentTxt', and replaces the current 'comments' array
          return post.comments = [{
            id: action.id,
    //new object is made with text entered (action.commentTxt) and added to 'post.comments' array
            commentTxt: action.commentTxt
          }, ...post.comments]
        }
      })

and would display it. And every time a new comment is added, a new would be rendered along with the previous comment objects in array. Would want to do something like the following:

      {
        this.props.post.comments.map((comment) => {
          return <Comment key={comment.id} comment={comment} actions={this.props.actions}/>
        })
      }

I heard mutating state directly is not recommended, so I would appreciate any guidance or insight on how to properly do so.

Answer 1

You might consider normalizing your data. So, instead of storing your structure like this:

posts: [{
  title: 'Some post',
  comments: [{
    text: 'Hey there'
  }]
}]

You'd store them like this:

posts: [{
  id: 1,
  title: 'Some post'
}]

comments: [{
  id: 4,
  postId: 1,
  text: 'Hey there'
}]

It's more of a pain at first, but allows a lot of flexibility.

Alternatively, you could modify your ADD_COMMENT reducer:

return posts.map(function(post) { 
  if (post.id !== action.id) {
    return post
  }


  return {
    ...post, 
    comments: [
      ...post.comments, 
      { 
        id: action.id, 
        commentTxt: action.commentTxt 
      }
    ]
  }
}

Note: In this last solution, there are no mutations. Don't know how it would perform with tons of comments, but I wouldn't pre-optimize for that scenario unless you have good reason.

Answer 2

As stated by Christopher Davies in his answer, you should normalize your state. Let's say we have a shape like this :

const exampleState = {
    posts: {
        '123': {
            id: '123',
            title: 'My first post',
            comments: []  // an array of comments ids
        },
        '456': {
            id: '456',
            title: 'My second post',
            comments: []  // an array of comments ids
        }
    },
    comments: {
        'abc': {
            id: 'abc',
            text: 'Lorem ipsum'
        },
        'def': {
            id: 'def',
            text: 'Dolor sit'
        },
        'ghi': {
            id: 'ghi',
            text: 'Amet conseguir'
        }
    }
}

Ok, now let's write some actions creators that creates action that will mutate the state :

const addPost = (post) => ({
    type: 'ADD_POST',
    post
})

const addComment = (postId, comment) => ({  // for the sake of example, let's say the "comment" object here is a comment object returned by some ajax request and having it's own id
    type: 'ADD_COMMENT',
    postId,
    comment
})

Then, you will need two reducers to handle the posts slice, and the comments slice :

const postsReducer = (posts = {}, action = {}) => {
    switch(action.type) {
        case 'ADD_POST':
            const id = getId(posts)
            return {
                ...posts,
                [id]: action.post
            }
        case 'ADD_COMMENT':
            return {
                ...posts.map(p => {
                    if (p.id == action.postId) {
                        return {
                            ...p,
                            comments: p.comments.concat([action.comment.id])
                        }
                    }
                    return p
                })
            }
        default:
            return state
    }
}

const commentsReducer = (comments = {}, action = {}) => {
    switch(action.type) {
        case 'ADD_COMMENT':
            return {
                ...comments,
                [action.comment.id]: action.comment
            }
        default:
            return state
    }
}

Let's also create some selectors to pick up data from the state :

const getPost = (state, id) => state.posts[id]

const getCommentsForPost = (state, id) => ({
    const commentsIds = state.posts[id].comments
    return state.comments.filter(c => commentsIds.includes(c.id))
})

Then, your components :

const PostLists = (posts) => (
    <ul>
        {posts.map(p) => <Post key={p} id={p} />}
    </ul>
)

PostLists.propTypes = {
    posts: React.PropTypes.arrayOf(React.PropTypes.string)  //just an id of posts
}


const Post = ({id, title, comments}) => (
    <li>
        {title}
        {comments.map(c) => <Comment key={c.id} {...c}/>}
    </li>
)

Post.propTypes = {
    id: React.PropTypes.string,
    comments: React.PropTypes.arrayOf(React.PropTypes.shape({
        id: React.PropTypes.string,
        text: React.PropTypes.text
    }))
}


const Comment = ({ id, text }) => (
    <p>{text}</p>
)

And now, the connected containers :

// the mapStateToProps if very simple here, we just extract posts ids from state
const ConnectedPostLists = connect(
    (state) => ({
        posts: Objects.keys(state.posts)
    })
)(PostLists)


// The ConnectedPost could be written naively using the component props passed as the second argument of mapStateToProps :
const ConnectedPost = connect(
    (state, { id }) => ({
        id,
        title: getPost(state, id).title,
        comments: getCommentsForPost(state, id)
    })
)(Post)

That is going to work BUT, if you have many posts, you will hit performance issues with the ConnectedPost component because mapStateToProps that depends on component own props will trigger a re-render of the connected component for any change in the state

So we should rewrite it like this :

// Since the post id is never intended to change for this particular post, we can write the ConnectedPost like this :
const ConnectedPost = connect(
    (_, { id}) => (state) => ({
        id,
        title: getPost(state, id).title,
        comments: getCommentsForPost(state, id)
    })
)

And voilà ! I didn't test this example, but I think it can help you to see in which direction you need to go.