JSON in Swift — Codable, JSONDecoder & JSONEncoder

Swift's Codable protocol (combining Encodable and Decodable) makes JSON serialisation straightforward and type-safe. The Foundation framework's JSONDecoder and JSONEncoder handle the heavy lifting with minimal boilerplate.

Decode JSON with Codable

Conform your struct or class to Codable and use JSONDecoder to parse a JSON string into a Swift type.

import Foundation

struct User: Codable {
    let id: Int
    let name: String
    let email: String
}

let json = """
{"id":1,"name":"Alice","email":"alice@example.com"}
""".data(using: .utf8)!

let decoder = JSONDecoder()
let user = try decoder.decode(User.self, from: json)
print(user.name) // Alice

Encode a struct to JSON

Use JSONEncoder to serialise a Codable value to a Data object, then convert to a String.

let user = User(id: 1, name: "Alice", email: "alice@example.com")

let encoder = JSONEncoder()
encoder.outputFormatting = .prettyPrinted

let data = try encoder.encode(user)
print(String(data: data, encoding: .utf8)!)

Customise keys with CodingKeys

Use a CodingKeys enum to map JSON snake_case keys to Swift camelCase property names.

struct User: Codable {
    let id: Int
    let firstName: String
    let lastName: String

    enum CodingKeys: String, CodingKey {
        case id
        case firstName = "first_name"
        case lastName  = "last_name"
    }
}

Handle optional fields

Declare optional properties with ? to handle missing or null JSON keys without throwing a decoding error.

struct Profile: Codable {
    let id: Int
    let bio: String?   // may be null or absent
    let website: URL?  // may be null or absent
}

// Missing "bio" and "website" keys decode fine:
let json = #"{"id": 42}"#.data(using: .utf8)!
let profile = try JSONDecoder().decode(Profile.self, from: json)
print(profile.bio)     // nil
print(profile.website) // nil

Date decoding strategies

JSONDecoder provides built-in strategies for decoding ISO 8601 dates, Unix timestamps, and custom formats.

struct Event: Codable {
    let name: String
    let date: Date
}

let decoder = JSONDecoder()

// ISO 8601 string: "2024-06-15T10:30:00Z"
decoder.dateDecodingStrategy = .iso8601

// Unix timestamp (seconds since epoch): 1718443800
decoder.dateDecodingStrategy = .secondsSince1970

// Custom format
let formatter = DateFormatter()
formatter.dateFormat = "yyyy-MM-dd"
decoder.dateDecodingStrategy = .formatted(formatter)

Decode a JSON array

Decode a top-level JSON array directly into a Swift array of Codable types.

let json = """
[
  {"id":1,"name":"Alice","email":"a@example.com"},
  {"id":2,"name":"Bob","email":"b@example.com"}
]
""".data(using: .utf8)!

let users = try JSONDecoder().decode([User].self, from: json)
print(users.count)       // 2
print(users[0].name)     // Alice

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Frequently Asked Questions

What is Codable in Swift?▾

Codable is a type alias for Encodable & Decodable. Conforming a struct or class to Codable lets Swift automatically synthesise JSON serialisation and deserialisation code when all stored properties are themselves Codable.

How do I decode JSON in Swift?▾

Create a Codable struct matching the JSON shape, convert the JSON string to Data with .data(using: .utf8), then call JSONDecoder().decode(MyType.self, from: data). The call throws a DecodingError if parsing fails.

How do I map snake_case JSON keys to camelCase Swift properties?▾

Either provide a CodingKeys enum with raw string values (e.g. case firstName = "first_name"), or set decoder.keyDecodingStrategy = .convertFromSnakeCase on your JSONDecoder to apply the conversion automatically.

How do I handle nullable or missing JSON fields in Swift?▾

Declare the property as Optional (String?, Int?, etc.). JSONDecoder treats a null JSON value or a missing key as nil rather than throwing an error.

How do I decode ISO 8601 dates in Swift?▾

Set decoder.dateDecodingStrategy = .iso8601 on your JSONDecoder. For Unix timestamps, use .secondsSince1970 or .millisecondsSince1970. For custom formats, pass a DateFormatter via .formatted(formatter).

JSON in Other Languages

JSON in Python →JSON in JavaScript →JSON in TypeScript →JSON in Go →JSON in Java →JSON in PHP →JSON in Ruby →JSON in C# →JSON in Rust →JSON in Kotlin →JSON in Dart →JSON in Scala →

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