Along with Swift 4 (Foundation) now it is natively supported in both ways, JSON string to an object - an object to JSON string. Please see Apple's documentation here JSONDecoder() and here JSONEncoder()

JSON String to Object

let jsonData = jsonString.data(using: .utf8)!
let decoder = JSONDecoder()
let myStruct = try! decoder.decode(myStruct.self, from: jsonData)
Swift Object to JSONString

let encoder = JSONEncoder()
encoder.outputFormatting = .prettyPrinted
let data = try! encoder.encode(myStruct)
print(String(data: data, encoding: .utf8)!)
You can find all details and examples here Ultimate Guide to JSON Parsing With Swift 4