JSON to Swift Struct

Codable is Swift's built-in protocol for encoding and decoding data. A struct or class that conforms to Codable (which combines Encodable and Decodable) can be serialized to JSON and deserialized from JSON using JSONEncoder and JSONDecoder, both part of the Foundation framework.

Codable is the standard for API calls in iOS, macOS, watchOS, and tvOS apps — and in server-side Swift with Vapor or Hummingbird. Writing Codable structs by hand for complex JSON responses is slow and error-prone. This tool generates all the structs from your JSON, complete with CodingKeys enums for snake_case-to-camelCase mapping.

Input JSON from an API:

{
  "user_id": 42,
  "display_name": "Alice",
  "is_premium": true,
  "account": { "plan": "pro", "expires_at": "2026-12-01" }
}

Generated Swift structs:

import Foundation

struct Account: Codable {
    let plan: String
    let expiresAt: String

    enum CodingKeys: String, CodingKey {
        case plan
        case expiresAt = "expires_at"
    }
}

struct Root: Codable {
    let userId: Int
    let displayName: String
    let isPremium: Bool
    let account: Account

    enum CodingKeys: String, CodingKey {
        case userId = "user_id"
        case displayName = "display_name"
        case isPremium = "is_premium"
        case account
    }
}

let (data, _) = try await URLSession.shared.data(from: url)
let root = try JSONDecoder().decode(Root.self, from: data)

Or use a custom decoder: let decoder = JSONDecoder(); decoder.keyDecodingStrategy = .convertFromSnakeCase — but manual CodingKeys gives you more control.