Solution

src/App.scss

@@ -1,3 +1,5 @@
+@use 'sass:math';
+
 .Sidebar {
   overflow: hidden;
   opacity: 0;
@@ -21,3 +23,48 @@
 .message-body {
   white-space: pre-line;
 }
+
+/* missed classes */
+
+@mixin tablet {
+  @media screen and (min-width: 769px), print {
+    @content;
+  }
+}
+
+$tile-spacing: 0.75rem !default;
+
+.tile {
+  align-items: stretch;
+  display: block;
+  flex: 1;
+  min-height: min-content;
+
+  &.is-ancestor {
+    margin-left: $tile-spacing * -1;
+    margin-right: $tile-spacing * -1;
+    margin-top: $tile-spacing * -1;
+    &:last-child {
+      margin-bottom: $tile-spacing * -1; }
+    &:not(:last-child) {
+      margin-bottom: $tile-spacing; } }
+  &.is-child {
+    margin: 0 !important; }
+  &.is-parent {
+    padding: $tile-spacing; }
+  &.is-vertical {
+    flex-direction: column;
+
+    & > .tile.is-child:not( :last-child ) {
+      margin-bottom: 1.5rem !important; } }
+
+  @include tablet {
+    &:not(.is-child) {
+      display: flex;
+    }
+
+    @for $i from 1 through 12 {
+      &.is-#{$i} {
+        flex: none;
+        width: (math.div($i, 12) * 100%); } } }
+  }

src/App.tsx

@@ -1,3 +1,4 @@
+import { useEffect, useState } from 'react';
 import classNames from 'classnames';
 
 import 'bulma/css/bulma.css';
@@ -9,52 +10,188 @@ import { PostDetails } from './components/PostDetails';
 import { UserSelector } from './components/UserSelector';
 import { Loader } from './components/Loader';
 
-export const App = () => (
-  <main className="section">
-    <div className="container">
-      <div className="tile is-ancestor">
-        <div className="tile is-parent">
-          <div className="tile is-child box is-success">
-            <div className="block">
-              <UserSelector />
-            </div>
+import { User } from './types/User';
+import { Post } from './types/Post';
+import { getUsers } from './api/user';
+import { getPosts } from './api/post';
+import { addComment, deleteComment, getComments } from './api/comment';
+import { Comment } from './types/Comment';
 
-            <div className="block" data-cy="MainContent">
-              <p data-cy="NoSelectedUser">No user selected</p>
+export const App = () => {
+  const [selectedUser, setSelectedUser] = useState<User | null>(null);
+  const [users, setUsers] = useState<User[]>([]);
+  const [isLoadingUsers, setIsLoadingUsers] = useState(false);
+  const [isLoadingPosts, setIsLoadingPosts] = useState(false);
+  const [isLoadingComments, setIsLoadingComments] = useState(false);
+  const [error, setError] = useState<string | null>(null);
+  const [selectedPost, setSelectedPost] = useState<Post | null>(null);
+  const [posts, setPosts] = useState<Post[]>([]);
+  const [comments, setComments] = useState<Comment[]>([]);
+  const [isFormOpen, setIsFormOpen] = useState(false);
 
-              <Loader />
+  useEffect(() => {
+    const fetchUsers = async () => {
+      setIsLoadingUsers(true);
+      try {
+        const usersData = await getUsers();
 
-              <div
-                className="notification is-danger"
-                data-cy="PostsLoadingError"
-              >
-                Something went wrong!
-              </div>
+        setUsers(usersData);
+        setError(null);
+      } catch (err) {
+        setError('Failed to load users');
+      } finally {
+        setIsLoadingUsers(false);
+      }
+    };
+
+    fetchUsers();
+  }, []);
+
+  useEffect(() => {
+    const fetchPosts = async () => {
+      setIsLoadingPosts(true);
+
+      try {
+        if (selectedUser) {
+          const userPosts = await getPosts(selectedUser.id);
+
+          setPosts(userPosts);
+          setError(null);
+        }
+      } catch (err) {
+        setError('Failed to load posts');
+      } finally {
+        setIsLoadingPosts(false);
+      }
+    };
+
+    fetchPosts();
+  }, [selectedUser]);
+
+  useEffect(() => {
+    const fetchComments = async () => {
+      setIsLoadingComments(true);
+      try {
+        if (selectedPost) {
+          const postComments = await getComments(selectedPost.id);
+
+          setComments(postComments);
+          setError(null);
+        }
+      } catch (err) {
+        setError('Failed to load comments');
+      } finally {
+        setIsLoadingComments(false);
+      }
+    };
+
+    fetchComments();
+  }, [selectedPost]);
 
-              <div className="notification is-warning" data-cy="NoPostsYet">
-                No posts yet
+  const handleDeleteComment = async (commentId: number) => {
+    try {
+      await deleteComment(commentId);
+      setComments(currentComments =>
+        currentComments.filter(comment => comment.id !== commentId),
+      );
+    } catch (err) {
+      setError('Unable to delete comment');
+    }
+  };
+
+  const handleAddComment = async (comment: Comment) => {
+    setIsLoadingComments(true);
+    try {
+      const newComment: Comment = await addComment({
+        postId: comment.postId,
+        name: comment.name,
+        email: comment.email,
+        body: comment.body,
+      });
+
+      setComments(prevComments => [...prevComments, newComment]);
+    } catch (err) {
+      setError('Unable to add a comment');
+    } finally {
+      setIsLoadingComments(false);
+    }
+  };
+
+  return (
+    <main className="section">
+      <div className="container">
+        <div className="tile is-ancestor">
+          <div className="tile is-parent">
+            <div className="tile is-child box is-success">
+              {(isLoadingUsers || isLoadingPosts) && <Loader />}
+              <div className="block">
+                <UserSelector
+                  selectedUser={selectedUser}
+                  users={users}
+                  onSelect={setSelectedUser}
+                />
+              </div>
+              <div className="block" data-cy="MainContent">
+                {selectedUser && !error && !isLoadingPosts ? (
+                  posts.length > 0 ? (
+                    <PostsList
+                      posts={posts}
+                      selectedPost={selectedPost}
+                      onOpen={setSelectedPost}
+                      onClose={() => {
+                        setSelectedPost(null);
+                      }}
+                    />
+                  ) : (
+                    <div
+                      className="notification is-warning"
+                      data-cy="NoPostsYet"
+                    >
+                      No posts yet
+                    </div>
+                  )
+                ) : (
+                  <p data-cy="NoSelectedUser">No user selected</p>
+                )}
               </div>
 
-              <PostsList />
+              {error && (
+                <div
+                  className="notification is-danger"
+                  data-cy="PostsLoadingError"
+                >
+                  {error}
+                </div>
+              )}
             </div>
           </div>
-        </div>
-
-        <div
-          data-cy="Sidebar"
-          className={classNames(
-            'tile',
-            'is-parent',
-            'is-8-desktop',
-            'Sidebar',
-            'Sidebar--open',
+          {selectedPost && (
+            <div
+              data-cy="Sidebar"
+              className={classNames(
+                'tile',
+                'is-parent',
+                'is-8-desktop',
+                'Sidebar',
+                'Sidebar--open',
+              )}
+            >
+              <div className="tile is-child box is-success ">
+                <PostDetails
+                  post={selectedPost}
+                  error={error}
+                  comments={comments}
+                  formOpen={setIsFormOpen}
+                  isFormOpen={isFormOpen}
+                  deleteComment={handleDeleteComment}
+                  addComment={handleAddComment}
+                  isLoadingComments={isLoadingComments}
+                />
+              </div>
+            </div>
           )}
-        >
-          <div className="tile is-child box is-success ">
-            <PostDetails />
-          </div>
         </div>
       </div>
-    </div>
-  </main>
-);
+    </main>
+  );
+};

src/api/comment.ts

@@ -0,0 +1,14 @@
+import { Comment, CommentData } from '../types/Comment';
+import { client } from '../utils/fetchClient';
+
+export const getComments = (postId: number) => {
+  return client.get<Comment[]>(`/comments?postId=${postId}`);
+};
+
+export function deleteComment(commentId: number) {
+  return client.delete(`/comments/${commentId}`);
+}
+
+export function addComment({ postId, name, email, body }: CommentData) {
+  return client.post<Comment>('/comments', { postId, name, email, body });
+}

src/api/post.ts

@@ -0,0 +1,6 @@
+import { Post } from '../types/Post';
+import { client } from '../utils/fetchClient';
+
+export const getPosts = (userId: number) => {
+  return client.get<Post[]>(`/posts?userId=${userId}`);
+};

src/api/user.ts

@@ -0,0 +1,6 @@
+import { User } from '../types/User';
+import { client } from '../utils/fetchClient';
+
+export const getUsers = () => {
+  return client.get<User[]>('/users');
+};

src/components/Loader/Loader.scss

@@ -1,6 +1,7 @@
 .Loader {
   display: flex;
   width: 100%;
+  margin-left: auto;
   justify-content: center;
   align-items: center;