extension Data {
    func printJson() {
        do {
            let object = try JSONSerialization.jsonObject(with: self)
            let jsonData = try JSONSerialization.data(withJSONObject: object, options: .prettyPrinted)
            let jsonString = String(data: jsonData, encoding: .utf8)
            print(jsonString ?? "nil")
        } catch {
            print("Not valid json format \\(self)")
        }
    }
}

Data를 String으로 만들때 쓰는 init 2개

print(String(decoding: data, as: UTF8.self))

print(String(data: b!, encoding: .utf8))