Generate Swift Structs
from JSON

Swift struct definitions in four steps

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Converting JSON to Swift Codable structs

Swift Codable protocol provides built-in JSON encoding and decoding. By defining a struct conforming to Codable, you can deserialize JSON with JSONDecoder without third-party libraries. This tool generates Swift struct definitions with proper types (String, Int, Double, Bool), optional handling (String?), and CodingKeys enums.

The converter follows Swift conventions: PascalCase struct names, camelCase properties, [Element] for arrays, and Optional<T> as T? for nullable fields. When JSON keys use snake_case, it generates a CodingKeys enum for mapping. The output works with JSONDecoder in iOS, macOS, watchOS, and tvOS.

{
  "user_name": "Alice",
  "age": 30,
  "roles": ["admin"],
  "bio": null
}

struct Root: Codable {
    let userName: String
    let age: Int
    let roles: [String]
    let bio: String?

    enum CodingKeys: String,
      CodingKey {
        case userName = "user_name"
        case age, roles, bio
    }
}

Get the most out of this tool

• Use JSONDecoder().keyDecodingStrategy = .convertFromSnakeCase instead of CodingKeys when all keys are snake_case.
• Mark fields as optional (String?) when the API may return null — non-optional fields cause decoding failure if missing.
• Swift structs with Codable work with both JSONDecoder and JSONEncoder — round-trip JSON without extra code.

Swift structs and Codable for JSON

Swift's Codable protocol (introduced in Swift 4) provides a built-in, type-safe JSON serialization and deserialization mechanism. Any Swift type that conforms to Codable (which combines Encodable and Decodable) can be serialized to JSON with JSONEncoder and deserialized from JSON with JSONDecoder. The converter generates Swift structs conforming to Codable, which is the idiomatic modern Swift approach to JSON handling — no third-party libraries required.

Swift structs are preferred over classes for JSON data models for several reasons. Structs are value types — they are copied rather than referenced, which prevents shared mutable state bugs. Structs are automatically synthesized with memberwise initializers. With Codable, a Swift struct requires only the Codable conformance declaration and property definitions — the compiler synthesizes the entire encoding and decoding logic automatically when all properties are themselves Codable.

The converter maps JSON types to their Swift equivalents: strings become String, integers become Int, floating-point numbers become Double, booleans become Bool, null values become Optional types (String?), arrays become [ElementType], and nested objects become nested struct types. The property names are converted from snake_case (common in JSON APIs) to camelCase (Swift convention) using CodingKeys enum or the JSONDecoder's keyDecodingStrategy.

CodingKeys is the Swift mechanism for mapping between JSON key names and Swift property names. When a JSON field is "first_name" but the Swift property should be firstName, you add a CodingKeys enum to the struct that maps between the two. The converter generates these CodingKeys automatically when JSON keys use snake_case. Alternatively, setting JSONDecoder().keyDecodingStrategy = .convertFromSnakeCase handles the conversion globally without explicit CodingKeys.

Optional properties in Swift JSON models deserve careful thought. If a JSON field might be absent in some responses, mark its type as Optional (e.g., String?). JSONDecoder treats absent keys as nil for optional properties. For non-optional properties, an absent key causes a DecodingError.keyNotFound error at runtime. Getting optionality right prevents hard-to-debug crashes in production iOS apps when an API returns fewer fields than expected.

SwiftUI and Combine make JSON decoding even more streamlined. URLSession's dataTaskPublisher in Combine can be chained with .decode(type:decoder:) to produce a publisher that emits your decoded Swift struct. Async/await in Swift 5.5+ enables data(from:delegate:) returning (Data, URLResponse), then JSONDecoder().decode(YourType.self, from: data). The generated structs from this tool plug directly into both patterns.

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