JSON in Mobile Apps — React Native, Swift & Kotlin

1struct User: Codable {
2    let id: Int
3    let name: String
4    let email: String
5    let bio: String?           // nullable field
6    let createdAt: Date
7    let settings: UserSettings
8
9    struct UserSettings: Codable {
10        let theme: String
11        let notifications: Bool
12    }
13
14    // Map JSON keys to Swift properties
15    enum CodingKeys: String, CodingKey {
16        case id, name, email, bio, settings
17        case createdAt = "created_at"  // snake_case → camelCase
18    }
19}

1func fetchUsers() async throws -> [User] {
2    let url = URL(string: "https://api.example.com/users")!
3    let (data, response) = try await URLSession.shared.data(from: url)
4
5    guard let httpResponse = response as? HTTPURLResponse,
6          httpResponse.statusCode == 200 else {
7        throw APIError.invalidResponse
8    }
9
10    let decoder = JSONDecoder()
11    decoder.dateDecodingStrategy = .iso8601
12    decoder.keyDecodingStrategy = .convertFromSnakeCase
13
14    return try decoder.decode([User].self, from: data)
15}
16
17// Usage in SwiftUI
18struct UserListView: View {
19    @State private var users: [User] = []
20
21    var body: some View {
22        List(users, id: \.id) { user in
23            VStack(alignment: .leading) {
24                Text(user.name).font(.headline)
25                Text(user.email).font(.caption)
26            }
27        }
28        .task {
29            users = (try? await fetchUsers()) ?? []
30        }
31    }
32}

1import kotlinx.serialization.Serializable
2import kotlinx.serialization.SerialName
3import kotlinx.serialization.json.Json
4
5@Serializable
6data class User(
7    val id: Int,
8    val name: String,
9    val email: String,
10    val bio: String? = null,                // nullable with default
11    @SerialName("created_at")
12    val createdAt: String,                  // ISO string
13    val settings: UserSettings = UserSettings()
14)
15
16@Serializable
17data class UserSettings(
18    val theme: String = "light",
19    val notifications: Boolean = true
20)
21
22// Configure parser
23val json = Json {
24    ignoreUnknownKeys = true       // Don't crash on extra fields
25    isLenient = true               // Accept unquoted values
26    coerceInputValues = true       // Coerce nulls to defaults
27    encodeDefaults = false         // Skip default values in output
28}

1import io.ktor.client.*
2import io.ktor.client.call.*
3import io.ktor.client.request.*
4import kotlinx.serialization.json.Json
5
6suspend fun fetchUsers(): List<User> {
7    val client = HttpClient()
8    val response: String = client.get("https://api.example.com/users").body()
9    return json.decodeFromString<List<User>>(response)
10}
11
12// Serialize to JSON
13val user = User(id = 1, name = "Alice", email = "[email protected]", createdAt = "2026-04-02T14:30:00Z")
14val jsonString = json.encodeToString(User.serializer(), user)

1import { z } from 'zod';
2
3const UserSchema = z.object({
4  id: z.number(),
5  name: z.string(),
6  email: z.string().email(),
7  bio: z.string().nullable(),
8  created_at: z.string().datetime(),
9  settings: z.object({
10    theme: z.enum(['light', 'dark']),
11    notifications: z.boolean(),
12  }),
13});
14
15type User = z.infer<typeof UserSchema>;
16
17async function fetchUsers(): Promise<User[]> {
18  const response = await fetch('https://api.example.com/users');
19  if (!response.ok) throw new Error(`HTTP ${response.status}`);
20
21  const data = await response.json();
22  return z.array(UserSchema).parse(data); // Validates at runtime
23}