.gitignore

@@ -1,5 +1,8 @@
 # Backend
 backend/target/
+backend/data/
+backend/*.mv.db
+backend/*.trace.db
 
 # Frontend
 frontend/node_modules/

README.md

@@ -6,6 +6,9 @@ innehåller två separata applikationer:
 - en backend byggd med Java 21, Spring Boot och Maven
 - en frontend byggd med React, TypeScript, Vite och pnpm
 
+Backend använder en lokal filbaserad H2-databas i `backend/data`. Databasschemat
+hanteras med Flyway. Databasfilerna är lokala och ignoreras av Git.
+
 ## Starta backend
 
 Backend startar på port 8080.
@@ -43,4 +46,3 @@ Frontend:
 cd frontend
 pnpm test
 ```
-

backend/pom.xml

@@ -26,6 +26,19 @@
             <groupId>org.springframework.boot</groupId>
             <artifactId>spring-boot-starter-web</artifactId>
         </dependency>
+        <dependency>
+            <groupId>org.springframework.boot</groupId>
+            <artifactId>spring-boot-starter-data-jpa</artifactId>
+        </dependency>
+        <dependency>
+            <groupId>org.springframework.boot</groupId>
+            <artifactId>spring-boot-starter-flyway</artifactId>
+        </dependency>
+        <dependency>
+            <groupId>com.h2database</groupId>
+            <artifactId>h2</artifactId>
+            <scope>runtime</scope>
+        </dependency>
         <dependency>
             <groupId>org.springframework.boot</groupId>
             <artifactId>spring-boot-starter-test</artifactId>
@@ -42,4 +55,3 @@
         </plugins>
     </build>
 </project>
-

backend/src/main/java/se/rubble/hemhub/api/ApiError.java

@@ -0,0 +1,5 @@
+package se.rubble.hemhub.api;
+
+public record ApiError(String code, String message) {
+}
+

backend/src/main/java/se/rubble/hemhub/api/ApiExceptionHandler.java

@@ -0,0 +1,30 @@
+package se.rubble.hemhub.api;
+
+import org.springframework.http.HttpStatus;
+import org.springframework.http.ResponseEntity;
+import org.springframework.web.bind.annotation.ExceptionHandler;
+import org.springframework.web.bind.annotation.RestControllerAdvice;
+
+import se.rubble.hemhub.user.InvalidUserNameException;
+import se.rubble.hemhub.user.UserNameAlreadyExistsException;
+
+@RestControllerAdvice
+public class ApiExceptionHandler {
+
+    @ExceptionHandler(InvalidUserNameException.class)
+    public ResponseEntity<ApiError> handleInvalidUserName() {
+        return ResponseEntity.badRequest()
+                .body(new ApiError(
+                        "INVALID_USER_NAME",
+                        "Namnet måste innehålla mellan 1 och 50 tecken."));
+    }
+
+    @ExceptionHandler(UserNameAlreadyExistsException.class)
+    public ResponseEntity<ApiError> handleDuplicateUserName() {
+        return ResponseEntity.status(HttpStatus.CONFLICT)
+                .body(new ApiError(
+                        "USER_NAME_ALREADY_EXISTS",
+                        "En användare med det namnet finns redan."));
+    }
+}
+

backend/src/main/java/se/rubble/hemhub/user/CreateUserRequest.java

@@ -0,0 +1,5 @@
+package se.rubble.hemhub.user;
+
+public record CreateUserRequest(String name) {
+}
+

backend/src/main/java/se/rubble/hemhub/user/InvalidUserNameException.java

@@ -0,0 +1,5 @@
+package se.rubble.hemhub.user;
+
+public class InvalidUserNameException extends RuntimeException {
+}
+

backend/src/main/java/se/rubble/hemhub/user/User.java

@@ -0,0 +1,48 @@
+package se.rubble.hemhub.user;
+
+import java.time.Instant;
+import java.util.UUID;
+
+import jakarta.persistence.Column;
+import jakarta.persistence.Entity;
+import jakarta.persistence.Id;
+import jakarta.persistence.Table;
+
+@Entity
+@Table(name = "app_user")
+class User {
+
+    @Id
+    private UUID id;
+
+    @Column(nullable = false, length = 50)
+    private String name;
+
+    @Column(name = "normalized_name", nullable = false, length = 150, unique = true)
+    private String normalizedName;
+
+    @Column(name = "created_at", nullable = false)
+    private Instant createdAt;
+
+    protected User() {
+    }
+
+    User(UUID id, String name, String normalizedName, Instant createdAt) {
+        this.id = id;
+        this.name = name;
+        this.normalizedName = normalizedName;
+        this.createdAt = createdAt;
+    }
+
+    UUID getId() {
+        return id;
+    }
+
+    String getName() {
+        return name;
+    }
+
+    Instant getCreatedAt() {
+        return createdAt;
+    }
+}

backend/src/main/java/se/rubble/hemhub/user/UserController.java

@@ -0,0 +1,34 @@
+package se.rubble.hemhub.user;
+
+import java.util.List;
+
+import org.springframework.http.HttpStatus;
+import org.springframework.web.bind.annotation.GetMapping;
+import org.springframework.web.bind.annotation.PostMapping;
+import org.springframework.web.bind.annotation.RequestBody;
+import org.springframework.web.bind.annotation.RequestMapping;
+import org.springframework.web.bind.annotation.ResponseStatus;
+import org.springframework.web.bind.annotation.RestController;
+
+@RestController
+@RequestMapping("/api/users")
+public class UserController {
+
+    private final UserService userService;
+
+    UserController(UserService userService) {
+        this.userService = userService;
+    }
+
+    @GetMapping
+    public List<UserResponse> findAll() {
+        return userService.findAll();
+    }
+
+    @PostMapping
+    @ResponseStatus(HttpStatus.CREATED)
+    public UserResponse create(@RequestBody(required = false) CreateUserRequest request) {
+        return userService.create(request == null ? null : request.name());
+    }
+}
+

backend/src/main/java/se/rubble/hemhub/user/UserNameAlreadyExistsException.java

@@ -0,0 +1,5 @@
+package se.rubble.hemhub.user;
+
+public class UserNameAlreadyExistsException extends RuntimeException {
+}
+

backend/src/main/java/se/rubble/hemhub/user/UserRepository.java

@@ -0,0 +1,11 @@
+package se.rubble.hemhub.user;
+
+import java.util.UUID;
+
+import org.springframework.data.jpa.repository.JpaRepository;
+
+interface UserRepository extends JpaRepository<User, UUID> {
+
+    boolean existsByNormalizedName(String normalizedName);
+}
+

backend/src/main/java/se/rubble/hemhub/user/UserResponse.java

@@ -0,0 +1,12 @@
+package se.rubble.hemhub.user;
+
+import java.time.Instant;
+import java.util.UUID;
+
+public record UserResponse(UUID id, String name, Instant createdAt) {
+
+    static UserResponse from(User user) {
+        return new UserResponse(user.getId(), user.getName(), user.getCreatedAt());
+    }
+}
+

backend/src/main/java/se/rubble/hemhub/user/UserService.java

@@ -0,0 +1,66 @@
+package se.rubble.hemhub.user;
+
+import java.time.Clock;
+import java.time.Instant;
+import java.util.Comparator;
+import java.util.List;
+import java.util.Locale;
+import java.util.UUID;
+
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.dao.DataIntegrityViolationException;
+import org.springframework.stereotype.Service;
+import org.springframework.transaction.annotation.Transactional;
+
+@Service
+class UserService {
+
+    private static final Comparator<User> BY_DISPLAY_NAME =
+            Comparator.comparing(User::getName, String.CASE_INSENSITIVE_ORDER)
+                    .thenComparing(User::getName)
+                    .thenComparing(User::getId);
+
+    private final UserRepository userRepository;
+    private final Clock clock;
+
+    @Autowired
+    UserService(UserRepository userRepository) {
+        this(userRepository, Clock.systemUTC());
+    }
+
+    UserService(UserRepository userRepository, Clock clock) {
+        this.userRepository = userRepository;
+        this.clock = clock;
+    }
+
+    @Transactional(readOnly = true)
+    List<UserResponse> findAll() {
+        return userRepository.findAll().stream()
+                .sorted(BY_DISPLAY_NAME)
+                .map(UserResponse::from)
+                .toList();
+    }
+
+    @Transactional
+    UserResponse create(String requestedName) {
+        String name = requestedName == null ? "" : requestedName.trim();
+
+        if (name.isEmpty() || name.codePointCount(0, name.length()) > 50) {
+            throw new InvalidUserNameException();
+        }
+
+        String normalizedName = name.toLowerCase(Locale.ROOT);
+
+        if (userRepository.existsByNormalizedName(normalizedName)) {
+            throw new UserNameAlreadyExistsException();
+        }
+
+        User user = new User(UUID.randomUUID(), name, normalizedName, Instant.now(clock));
+
+        try {
+            return UserResponse.from(userRepository.saveAndFlush(user));
+        } catch (DataIntegrityViolationException exception) {
+            throw new UserNameAlreadyExistsException();
+        }
+    }
+}

backend/src/main/resources/application.properties

@@ -0,0 +1,7 @@
+spring.datasource.url=jdbc:h2:file:./data/hemhub;MODE=PostgreSQL;DATABASE_TO_LOWER=TRUE;DEFAULT_NULL_ORDERING=HIGH
+spring.datasource.username=sa
+spring.datasource.password=
+spring.jpa.hibernate.ddl-auto=validate
+spring.jpa.open-in-view=false
+spring.flyway.enabled=true
+

backend/src/main/resources/db/migration/V1__create_users.sql

@@ -0,0 +1,7 @@
+CREATE TABLE app_user (
+    id UUID PRIMARY KEY,
+    name VARCHAR(50) NOT NULL,
+    normalized_name VARCHAR(150) NOT NULL,
+    created_at TIMESTAMP WITH TIME ZONE NOT NULL,
+    CONSTRAINT uk_app_user_normalized_name UNIQUE (normalized_name)
+);

backend/src/test/java/se/rubble/hemhub/user/UserApiTest.java

@@ -0,0 +1,108 @@
+package se.rubble.hemhub.user;
+
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.boot.test.context.SpringBootTest;
+import org.springframework.http.MediaType;
+import org.springframework.test.web.servlet.MockMvc;
+import org.springframework.test.web.servlet.setup.MockMvcBuilders;
+import org.springframework.web.context.WebApplicationContext;
+
+import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.get;
+import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.post;
+import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.jsonPath;
+import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.status;
+
+@SpringBootTest
+class UserApiTest {
+
+    @Autowired
+    private WebApplicationContext context;
+
+    @Autowired
+    private UserRepository userRepository;
+
+    private MockMvc mockMvc;
+
+    @BeforeEach
+    void setUp() {
+        userRepository.deleteAll();
+        mockMvc = MockMvcBuilders.webAppContextSetup(context).build();
+    }
+
+    @Test
+    void listsNoUsersWhenDatabaseIsEmpty() throws Exception {
+        mockMvc.perform(get("/api/users"))
+                .andExpect(status().isOk())
+                .andExpect(jsonPath("$").isEmpty());
+    }
+
+    @Test
+    void createsAndListsUserWithTrimmedName() throws Exception {
+        mockMvc.perform(post("/api/users")
+                        .contentType(MediaType.APPLICATION_JSON)
+                        .content("""
+                                {"name": "  Urban  "}
+                                """))
+                .andExpect(status().isCreated())
+                .andExpect(jsonPath("$.id").isString())
+                .andExpect(jsonPath("$.name").value("Urban"))
+                .andExpect(jsonPath("$.createdAt").isString())
+                .andExpect(jsonPath("$.normalizedName").doesNotExist());
+
+        mockMvc.perform(get("/api/users"))
+                .andExpect(status().isOk())
+                .andExpect(jsonPath("$[0].name").value("Urban"));
+    }
+
+    @Test
+    void rejectsInvalidNames() throws Exception {
+        mockMvc.perform(post("/api/users")
+                        .contentType(MediaType.APPLICATION_JSON)
+                        .content("""
+                                {"name": "   "}
+                                """))
+                .andExpect(status().isBadRequest())
+                .andExpect(jsonPath("$.code").value("INVALID_USER_NAME"));
+
+        mockMvc.perform(post("/api/users")
+                        .contentType(MediaType.APPLICATION_JSON)
+                        .content("{\"name\": \"%s\"}".formatted("a".repeat(51))))
+                .andExpect(status().isBadRequest())
+                .andExpect(jsonPath("$.code").value("INVALID_USER_NAME"));
+    }
+
+    @Test
+    void rejectsDuplicateNameIgnoringCase() throws Exception {
+        createUser("Urban");
+
+        mockMvc.perform(post("/api/users")
+                        .contentType(MediaType.APPLICATION_JSON)
+                        .content("""
+                                {"name": "urban"}
+                                """))
+                .andExpect(status().isConflict())
+                .andExpect(jsonPath("$.code").value("USER_NAME_ALREADY_EXISTS"));
+    }
+
+    @Test
+    void listsUsersSortedByDisplayName() throws Exception {
+        createUser("Urban");
+        createUser("Anna");
+        createUser("Bertil");
+
+        mockMvc.perform(get("/api/users"))
+                .andExpect(status().isOk())
+                .andExpect(jsonPath("$[0].name").value("Anna"))
+                .andExpect(jsonPath("$[1].name").value("Bertil"))
+                .andExpect(jsonPath("$[2].name").value("Urban"));
+    }
+
+    private void createUser(String name) throws Exception {
+        mockMvc.perform(post("/api/users")
+                        .contentType(MediaType.APPLICATION_JSON)
+                        .content("{\"name\": \"%s\"}".formatted(name)))
+                .andExpect(status().isCreated());
+    }
+}

backend/src/test/resources/application.properties

@@ -0,0 +1,7 @@
+spring.datasource.url=jdbc:h2:mem:hemhub-test;MODE=PostgreSQL;DATABASE_TO_LOWER=TRUE;DEFAULT_NULL_ORDERING=HIGH;DB_CLOSE_DELAY=-1
+spring.datasource.username=sa
+spring.datasource.password=
+spring.jpa.hibernate.ddl-auto=validate
+spring.jpa.open-in-view=false
+spring.flyway.enabled=true
+

frontend/src/App.test.tsx

@@ -1,21 +1,145 @@
-import { render, screen } from '@testing-library/react'
+import { cleanup, fireEvent, render, screen } from '@testing-library/react'
-import { afterEach, expect, test, vi } from 'vitest'
+import { afterEach, beforeEach, expect, test, vi } from 'vitest'
 import App from './App'
 
+const users = [
+  {
+    id: 'd56b54dd-31b0-4d71-8a10-82464be59a61',
+    name: 'Urban',
+    createdAt: '2026-07-24T10:15:30Z',
+  },
+  {
+    id: '2213a673-c859-462b-a031-7b68b6f01c73',
+    name: 'Anna',
+    createdAt: '2026-07-24T10:16:30Z',
+  },
+]
+
+beforeEach(() => {
+  window.localStorage.clear()
+})
+
 afterEach(() => {
+  cleanup()
   vi.restoreAllMocks()
 })
 
-test('visar sidans rubrik', () => {
+test('tom användarlista visar Skapa användare', async () => {
-  vi.spyOn(globalThis, 'fetch').mockResolvedValue(
+  mockJsonResponse([])
-    new Response(JSON.stringify({ status: 'UP' }), {
+
-      status: 200,
+  render(<App />)
+
+  expect(await screen.findByRole('heading', { name: 'Skapa användare' })).toBeInTheDocument()
+})
+
+test('befintliga användare utan lokalt val visar Vem är du', async () => {
+  mockJsonResponse(users)
+
+  render(<App />)
+
+  expect(await screen.findByRole('heading', { name: 'Vem är du?' })).toBeInTheDocument()
+  expect(screen.getByRole('button', { name: 'Urban' })).toBeInTheDocument()
+  expect(screen.getByRole('button', { name: 'Anna' })).toBeInTheDocument()
+})
+
+test('val av användare lagrar id och visar startsidan', async () => {
+  mockJsonResponse(users)
+  render(<App />)
+
+  fireEvent.click(await screen.findByRole('button', { name: 'Urban' }))
+
+  expect(screen.getByRole('heading', { name: 'Hej Urban' })).toBeInTheDocument()
+  expect(window.localStorage.getItem('hemhub.activeUserId')).toBe(users[0].id)
+})
+
+test('skapad användare blir automatiskt aktiv', async () => {
+  const createdUser = users[0]
+  const fetchMock = vi.spyOn(globalThis, 'fetch')
+  fetchMock.mockResolvedValueOnce(jsonResponse([]))
+  fetchMock.mockResolvedValueOnce(jsonResponse(createdUser, 201))
+
+  render(<App />)
+
+  fireEvent.change(await screen.findByLabelText('Namn'), { target: { value: '  Urban  ' } })
+  fireEvent.click(screen.getByRole('button', { name: 'Skapa användare' }))
+
+  expect(await screen.findByRole('heading', { name: 'Hej Urban' })).toBeInTheDocument()
+  expect(window.localStorage.getItem('hemhub.activeUserId')).toBe(createdUser.id)
+  expect(fetchMock).toHaveBeenLastCalledWith('/api/users', {
+    method: 'POST',
     headers: { 'Content-Type': 'application/json' },
-    }),
+    body: JSON.stringify({ name: 'Urban' }),
+  })
+})
+
+test('skapandefel visas utan att namnet försvinner', async () => {
+  const fetchMock = vi.spyOn(globalThis, 'fetch')
+  fetchMock.mockResolvedValueOnce(jsonResponse([]))
+  fetchMock.mockResolvedValueOnce(
+    jsonResponse(
+      {
+        code: 'USER_NAME_ALREADY_EXISTS',
+        message: 'En användare med det namnet finns redan.',
+      },
+      409,
+    ),
   )
 
   render(<App />)
 
-  expect(screen.getByRole('heading', { name: 'HemHub' })).toBeInTheDocument()
+  const input = await screen.findByLabelText('Namn')
+  fireEvent.change(input, { target: { value: 'Urban' } })
+  fireEvent.click(screen.getByRole('button', { name: 'Skapa användare' }))
+
+  expect(await screen.findByRole('alert')).toHaveTextContent(
+    'En användare med det namnet finns redan.',
+  )
+  expect(input).toHaveValue('Urban')
 })
 
+test('Byt användare rensar valt id och visar användarvalet', async () => {
+  window.localStorage.setItem('hemhub.activeUserId', users[0].id)
+  mockJsonResponse(users)
+  render(<App />)
+
+  fireEvent.click(await screen.findByRole('button', { name: 'Byt användare' }))
+
+  expect(screen.getByRole('heading', { name: 'Vem är du?' })).toBeInTheDocument()
+  expect(window.localStorage.getItem('hemhub.activeUserId')).toBeNull()
+})
+
+test('hämtningsfel visas som fel och kan återförsökas', async () => {
+  const fetchMock = vi.spyOn(globalThis, 'fetch')
+  fetchMock.mockRejectedValueOnce(new Error('Nätverksfel'))
+  fetchMock.mockResolvedValueOnce(jsonResponse(users))
+  render(<App />)
+
+  expect(await screen.findByText('Kunde inte ansluta till HemHub.')).toBeInTheDocument()
+  expect(screen.queryByRole('heading', { name: 'Skapa användare' })).not.toBeInTheDocument()
+
+  fireEvent.click(screen.getByRole('button', { name: 'Försök igen' }))
+
+  expect(await screen.findByRole('heading', { name: 'Vem är du?' })).toBeInTheDocument()
+  expect(fetchMock).toHaveBeenCalledTimes(2)
+})
+
+test('ogiltigt lagrat användar-id rensas', async () => {
+  window.localStorage.setItem('hemhub.activeUserId', 'finns-inte')
+  mockJsonResponse(users)
+
+  render(<App />)
+
+  expect(await screen.findByRole('heading', { name: 'Vem är du?' })).toBeInTheDocument()
+  expect(window.localStorage.getItem('hemhub.activeUserId')).toBeNull()
+})
+
+function mockJsonResponse(body: unknown, status = 200) {
+  return vi.spyOn(globalThis, 'fetch').mockResolvedValue(jsonResponse(body, status))
+}
+
+function jsonResponse(body: unknown, status = 200) {
+  return new Response(JSON.stringify(body), {
+    status,
+    headers: { 'Content-Type': 'application/json' },
+  })
+}

frontend/src/App.tsx

@@ -1,48 +1,215 @@
-import { useEffect, useState } from 'react'
+import { FormEvent, useEffect, useRef, useState } from 'react'
 
-type HealthResponse = {
+type User = {
-  status: string
+  id: string
+  name: string
+  createdAt: string
 }
 
-function App() {
+type ApiError = {
-  const [backendStatus, setBackendStatus] = useState<string | null>(null)
+  code?: string
-  const [hasError, setHasError] = useState(false)
+  message?: string
+}
+
+const ACTIVE_USER_KEY = 'hemhub.activeUserId'
+
+function App() {
+  const [users, setUsers] = useState<User[]>([])
+  const [activeUser, setActiveUser] = useState<User | null>(null)
+  const [loadState, setLoadState] = useState<'loading' | 'ready' | 'error'>('loading')
+  const [showCreateUser, setShowCreateUser] = useState(false)
+
+  const loadUsers = async () => {
+    setLoadState('loading')
 
-  useEffect(() => {
-    const loadHealth = async () => {
     try {
-        const response = await fetch('/api/health')
+      const response = await fetch('/api/users')
 
       if (!response.ok) {
-          throw new Error(`Backend svarade med status ${response.status}`)
+        throw new Error('Kunde inte hämta användare')
       }
 
-        const health = (await response.json()) as HealthResponse
+      const loadedUsers = (await response.json()) as User[]
-        setBackendStatus(health.status)
+      const storedUserId = window.localStorage.getItem(ACTIVE_USER_KEY)
+      const storedUser = loadedUsers.find((user) => user.id === storedUserId) ?? null
+
+      if (storedUserId && !storedUser) {
+        window.localStorage.removeItem(ACTIVE_USER_KEY)
+      }
+
+      setUsers(loadedUsers)
+      setActiveUser(storedUser)
+      setShowCreateUser(loadedUsers.length === 0)
+      setLoadState('ready')
     } catch {
-        setHasError(true)
+      setLoadState('error')
     }
   }
 
-    void loadHealth()
+  useEffect(() => {
+    void loadUsers()
   }, [])
 
-  let statusMessage = 'Kontrollerar backend…'
+  const selectUser = (user: User) => {
+    window.localStorage.setItem(ACTIVE_USER_KEY, user.id)
+    setActiveUser(user)
+  }
 
-  if (hasError) {
+  const switchUser = () => {
-    statusMessage = 'Backend kunde inte nås'
+    window.localStorage.removeItem(ACTIVE_USER_KEY)
-  } else if (backendStatus) {
+    setActiveUser(null)
-    statusMessage = `Backend: ${backendStatus}`
+    setShowCreateUser(false)
+  }
+
+  if (loadState === 'loading') {
+    return <main>Laddar HemHub…</main>
+  }
+
+  if (loadState === 'error') {
+    return (
+      <main>
+        <h1>HemHub</h1>
+        <p>Kunde inte ansluta till HemHub.</p>
+        <button type="button" onClick={() => void loadUsers()}>
+          Försök igen
+        </button>
+      </main>
+    )
+  }
+
+  if (activeUser) {
+    return (
+      <main>
+        <h1>HemHub</h1>
+        <h2>Hej {activeUser.name}</h2>
+        <p>Du är vald som aktiv användare.</p>
+        <button type="button" onClick={switchUser}>
+          Byt användare
+        </button>
+      </main>
+    )
+  }
+
+  if (showCreateUser) {
+    return (
+      <CreateUserForm
+        hasExistingUsers={users.length > 0}
+        onCancel={() => setShowCreateUser(false)}
+        onCreated={(user) => {
+          setUsers((currentUsers) => [...currentUsers, user])
+          selectUser(user)
+        }}
+      />
+    )
   }
 
   return (
     <main>
       <h1>HemHub</h1>
-      <p>Frontend har startat.</p>
+      <h2>Vem är du?</h2>
-      <p aria-live="polite">{statusMessage}</p>
+      <div className="user-list">
+        {users.map((user) => (
+          <button type="button" key={user.id} onClick={() => selectUser(user)}>
+            {user.name}
+          </button>
+        ))}
+      </div>
+      <button type="button" className="secondary" onClick={() => setShowCreateUser(true)}>
+        Skapa användare
+      </button>
+    </main>
+  )
+}
+
+type CreateUserFormProps = {
+  hasExistingUsers: boolean
+  onCancel: () => void
+  onCreated: (user: User) => void
+}
+
+function CreateUserForm({ hasExistingUsers, onCancel, onCreated }: CreateUserFormProps) {
+  const [name, setName] = useState('')
+  const [error, setError] = useState('')
+  const [isSubmitting, setIsSubmitting] = useState(false)
+  const isSubmittingRef = useRef(false)
+
+  const submit = async (event: FormEvent<HTMLFormElement>) => {
+    event.preventDefault()
+
+    if (isSubmittingRef.current) {
+      return
+    }
+
+    const trimmedName = name.trim()
+
+    if (!trimmedName || [...trimmedName].length > 50) {
+      setError('Namnet måste innehålla mellan 1 och 50 tecken.')
+      return
+    }
+
+    setError('')
+    isSubmittingRef.current = true
+    setIsSubmitting(true)
+
+    try {
+      const response = await fetch('/api/users', {
+        method: 'POST',
+        headers: { 'Content-Type': 'application/json' },
+        body: JSON.stringify({ name: trimmedName }),
+      })
+
+      if (!response.ok) {
+        const apiError = (await response.json().catch(() => ({}))) as ApiError
+
+        if (
+          apiError.code === 'INVALID_USER_NAME' ||
+          apiError.code === 'USER_NAME_ALREADY_EXISTS'
+        ) {
+          setError(apiError.message ?? 'Det gick inte att skapa användaren.')
+        } else {
+          setError('Det gick inte att skapa användaren. Försök igen.')
+        }
+        return
+      }
+
+      const createdUser = (await response.json()) as User
+      onCreated(createdUser)
+    } catch {
+      setError('Det gick inte att skapa användaren. Försök igen.')
+    } finally {
+      isSubmittingRef.current = false
+      setIsSubmitting(false)
+    }
+  }
+
+  return (
+    <main>
+      <h1>HemHub</h1>
+      <h2>Skapa användare</h2>
+      <form onSubmit={(event) => void submit(event)}>
+        <label htmlFor="user-name">Namn</label>
+        <input
+          id="user-name"
+          value={name}
+          disabled={isSubmitting}
+          onChange={(event) => setName(event.target.value)}
+        />
+        {error && (
+          <p className="error" role="alert">
+            {error}
+          </p>
+        )}
+        <button type="submit" disabled={isSubmitting}>
+          {isSubmitting ? 'Skapar…' : 'Skapa användare'}
+        </button>
+        {hasExistingUsers && (
+          <button type="button" className="secondary" disabled={isSubmitting} onClick={onCancel}>
+            Tillbaka
+          </button>
+        )}
+      </form>
     </main>
   )
 }
 
 export default App
-

frontend/src/styles.css

@@ -21,3 +21,53 @@ h1 {
   margin-top: 0;
 }
 
+button,
+input {
+  font: inherit;
+}
+
+button {
+  padding: 0.65rem 1rem;
+  border: 0;
+  border-radius: 0.4rem;
+  color: white;
+  background: #2563eb;
+  cursor: pointer;
+}
+
+button:disabled,
+input:disabled {
+  cursor: not-allowed;
+  opacity: 0.65;
+}
+
+.secondary {
+  margin-top: 1rem;
+  color: #1f2937;
+  background: #e5e7eb;
+}
+
+.user-list {
+  display: flex;
+  flex-wrap: wrap;
+  gap: 0.75rem;
+}
+
+form {
+  display: flex;
+  flex-direction: column;
+  align-items: flex-start;
+  gap: 0.75rem;
+}
+
+input {
+  box-sizing: border-box;
+  width: 100%;
+  padding: 0.6rem;
+  border: 1px solid #9ca3af;
+  border-radius: 0.4rem;
+}
+
+.error {
+  color: #b91c1c;
+}

frontend/src/test/setup.ts

@@ -1,2 +1,27 @@
 import '@testing-library/jest-dom/vitest'
 
+const storedValues = new Map<string, string>()
+
+Object.defineProperty(window, 'localStorage', {
+  configurable: true,
+  value: {
+    get length() {
+      return storedValues.size
+    },
+    clear() {
+      storedValues.clear()
+    },
+    getItem(key: string) {
+      return storedValues.get(key) ?? null
+    },
+    key(index: number) {
+      return [...storedValues.keys()][index] ?? null
+    },
+    removeItem(key: string) {
+      storedValues.delete(key)
+    },
+    setItem(key: string, value: string) {
+      storedValues.set(key, String(value))
+    },
+  } satisfies Storage,
+})

frontend/vite.config.ts

@@ -11,6 +11,11 @@ export default defineConfig({
   },
   test: {
     environment: 'jsdom',
+    environmentOptions: {
+      jsdom: {
+        url: 'http://localhost',
+      },
+    },
     setupFiles: './src/test/setup.ts',
   },
 })