Serialize JSON string that contains escaped (backslash and double quote) Swift return Badly formed object

First of all if you wrap the JSON in the literal string syntax of Swift 4 you have to escape the backslashes.

let jsonStr = """
{
"status": "success",
"data": "{\\"name\\":\\"asd\\",\\"address\\":\\"Street 1st\\"}"
}
"""

You got nested JSON. The value for key data is another JSON string which must be deserialized separately

let jsonData = Data(jsonStr.utf8)

do {
    if let object = try JSONSerialization.jsonObject(with: jsonData) as? [String:String] {
        print(object)
        if let dataString = object["data"] as? String {
            let dataStringData = Data(dataString.utf8)
            let dataObject = try JSONSerialization.jsonObject(with: dataStringData) as? [String:String]
            print(dataObject)
        }
    }
} catch {
    print(error)
}

Or – with a bit more effort but – much more comfortable with the (De)Codable protocol

struct Response : Decodable {

    private enum CodingKeys : String, CodingKey { case status, data }

    let status : String
    let person : Person

    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        status = try container.decode(String.self, forKey: .status)
        let dataString = try container.decode(String.self, forKey: .data)
        person = try JSONDecoder().decode(Person.self, from: Data(dataString.utf8))
    }
}

struct Person : Decodable {
    let name, address : String
}

---

let jsonStr = """
{
"status": "success",
"data": "{\\"name\\":\\"asd\\",\\"address\\":\\"Street 1st\\"}"
}
"""
let jsonData = Data(jsonStr.utf8)

do {
    let result = try JSONDecoder().decode(Response.self, from: jsonData)
    print(result)
} catch {
    print(error)
}