Swift Academy · Lesson

Codable: Encoding and Decoding JSON

Conforming to Codable, custom CodingKeys and nested container decoding.

Lesson 2 of 413 steps

Codable: Encoding and Decoding JSON is a free Swift Academy lesson on CoddyKit. This is lesson 2 of 4. Any 3 lessons of this learning path are free to read in full — after that, CoddyKit PRO unlocks every lesson, plus hands-on practice with a built-in code editor and a 24/7 AI tutor. It is part of the Swift Academy learning path, and your progress syncs across the web and the CoddyKit app. The Swift Academy course includes 4 lessons in total.

Codable Protocol

Codable is a type alias for Encodable & Decodable. Conforming types can be encoded to and decoded from JSON automatically.

struct User: Codable {
  var name: String
  var age: Int
}

Automatic Synthesis

Swift synthesizes Codable implementations when all stored properties are themselves Codable.

struct Product: Codable {
  var id: Int
  var title: String
  var price: Double
  // init(from:) and encode(to:) are auto-generated
}

Decoding JSON

Use JSONDecoder to decode Data into your Decodable type.

let json = #"{"name":"Alice","age":30}"#.data(using: .utf8)!
let user = try JSONDecoder().decode(User.self, from: json)
print(user.name) // Alice

Encoding to JSON

Use JSONEncoder to encode a Codable value into Data.

let user = User(name: "Bob", age: 25)
let data = try JSONEncoder().encode(user)
print(String(data: data, encoding: .utf8)!)

Custom CodingKeys

Use a CodingKeys enum to map between Swift property names and JSON keys.

struct Article: Codable {
  var createdAt: Date
  enum CodingKeys: String, CodingKey {
    case createdAt = "created_at"
  }
}

Date Decoding Strategies

Configure JSONDecoder.dateDecodingStrategy to parse dates in various formats.

let decoder = JSONDecoder()
decoder.dateDecodingStrategy = .iso8601
let article = try decoder.decode(Article.self, from: data)

Nested Containers

Decode nested JSON objects by defining nested Codable structs or using keyed containers manually.

struct Response: Codable {
  struct Data: Codable {
    var users: [User]
  }
  var data: Data
}
let resp = try JSONDecoder().decode(Response.self, from: json)

Custom Decode Logic

Override init(from:) for non-standard JSON structures or to apply transformations during decode.

struct Rating: Codable {
  var stars: Int
  init(from decoder: Decoder) throws {
    let container = try decoder.singleValueContainer()
    let raw = try container.decode(Double.self)
    stars = Int(raw.rounded())
  }
  func encode(to encoder: Encoder) throws {
    var container = encoder.singleValueContainer()
    try container.encode(Double(stars))
  }
}

keyDecodingStrategy

Use .convertFromSnakeCase to automatically convert JSON snake_case keys to Swift camelCase properties.

let decoder = JSONDecoder()
decoder.keyDecodingStrategy = .convertFromSnakeCase
let user = try decoder.decode(User.self, from: data)
// "first_name" → firstName

Encoding Options

Configure JSONEncoder.outputFormatting for pretty-printed or sorted output.

let encoder = JSONEncoder()
encoder.outputFormatting = [.prettyPrinted, .sortedKeys]
let data = try encoder.encode(product)

Handling Optional Fields

Mark properties as Optional to handle missing JSON keys gracefully without throwing.

struct User: Codable {
  var name: String
  var bio: String?  // nil if "bio" key is absent from JSON
}

Quick Check

Which JSONDecoder strategy automatically maps JSON snake_case keys to Swift camelCase property names?

Answer it in the interactive lesson

Lesson Recap

Conform to Codable for automatic JSON encode/decode. Use CodingKeys to remap property names, keyDecodingStrategy = .convertFromSnakeCase for APIs with snake_case, custom init(from:) for non-standard JSON, and dateDecodingStrategy for date parsing.

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Frequently Asked Questions

Is the “Codable: Encoding and Decoding JSON” lesson free?

Yes — any 3 lessons of the Swift Academy learning path, including “Codable: Encoding and Decoding JSON”, are free to read in full here on the web. After that, CoddyKit PRO unlocks every lesson, plus interactive practice with a built-in code editor and a 24/7 AI tutor. The Swift Academy course includes 4 lessons in total.

What will I learn in “Codable: Encoding and Decoding JSON”?

Conforming to Codable, custom CodingKeys and nested container decoding. You practise Swift Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Swift Academy?

No prior experience is required. Swift Academy on CoddyKit is structured for beginners through advanced learners, so you can start here or from the beginning and move at your own pace. This is lesson 2 of 4.

How long does the “Codable: Encoding and Decoding JSON” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Swift Academy lesson?

Yes. Every Swift Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

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