Modification of https://gist.github.com/itaiferber/d26cf93355f3415f4f684510f21ca202 to be able to explore contents of JSON.

JSON.swift

import Foundation

public enum JSON : Codable {
    case null
    case boolean(Bool)
    case number(Double)
    case string(String)
    case array([JSON])
    case dictionary([String : JSON])
    
    public init(from decoder: Decoder) throws {
        let container = try decoder.singleValueContainer()
        if container.decodeNil() {
            self = .null
        } else if let bool = try container.decodeIfMatched(Bool.self) {
            self = .boolean(bool)
        } else if let double = try container.decodeIfMatched(Double.self) {
            self = .number(double)
        } else if let string = try container.decodeIfMatched(String.self) {
            self = .string(string)
        } else if let array = try container.decodeIfMatched([JSON].self) {
            self = .array(array)
        } else if let dictionary = try container.decodeIfMatched([String : JSON].self) {
            self = .dictionary(dictionary)
        } else {
            let context = DecodingError.Context(codingPath: decoder.codingPath,
                                                debugDescription: "The type is incompatible with JSON.")
            throw DecodingError.typeMismatch(JSON.self, context)
        }
    }
    
    public func encode(to encoder: Encoder) throws {
        var container = encoder.singleValueContainer()
        
        switch self {
        case .null: try container.encodeNil()
        case .boolean(let boolean): try container.encode(boolean)
        case .number(let number): try container.encode(number)
        case .string(let string): try container.encode(string)
        case .array(let array): try container.encode(array)
        case .dictionary(let dictionary): try container.encode(dictionary)
        }
    }
    
    var isNull: Bool {
        if case .null = self {
            return true
        }
        return false
    }
    
    var number: Double? {
        if case .number(let value) = self {
            return value
        }
        return nil
    }
    
    var boolean: Bool? {
        if case .boolean(let value) = self {
            return value
        }
        return nil
    }
    
    var string: String? {
        if case .string(let value) = self {
            return value
        }
        return nil
    }
    
    var array: [JSON]? {
        if case .array(let value) = self {
            return value
        }
        return nil
    }
    
    var dictionary: [String : JSON]? {
        if case .dictionary(let value) = self {
            return value
        }
        return nil
    }
    
    subscript(key: String) -> JSON? {
        return dictionary?[key]
    }
    
    subscript(index: Int) -> JSON? {
        return array.flatMap { index < $0.endIndex ? $0[index] : nil }
    }
    
    var value: Any? {
        switch self {
        case .null: return nil
        case .boolean(let boolean): return boolean
        case .number(let number): return number
        case .string(let string): return string
        case .array(let array): return array.compactMap { $0.value }
        case .dictionary(let dictionary): return dictionary.compactMapValues { $0.value }
        }
    }
}

private extension SingleValueDecodingContainer {
    func decodeIfMatched<T : Decodable>(_ type: T.Type) throws -> T? {
        do {
            return try self.decode(T.self)
        } catch DecodingError.typeMismatch {
            return nil
        }
    }
}

Playground.swift

extension JSON {
    var playgroundDescription: String {
        return String(describing: self)
            .replacingOccurrences(of: "__lldb_expr_\\d+\\.", with: "", options: .regularExpression, range: nil)
            .replacingOccurrences(of: "\"", with: "")
    }
}

let json = """
{
  "firstName": "John",
  "lastName": "Smith",
  "isAlive": true,
  "age": 27,
  "address": {
    "streetAddress": "21 2nd Street",
    "city": "New York",
    "state": "NY",
    "postalCode": "10021-3100"
  },
  "phoneNumbers": [
    {
      "type": "home",
      "number": "212 555-1234"
    },
    {
      "type": "office",
      "number": "646 555-4567"
    },
    {
      "type": "mobile",
      "number": "123 456-7890"
    }
  ],
  "children": [],
  "spouse": null
}
""".data(using: .utf8)!

let decoded = try JSONDecoder().decode(JSON.self, from: json)
print(decoded["phoneNumbers"]?[1]?["number"]?.string) // Optional("646 555-4567")
print(decoded["phoneNumbers"]?.playgroundDescription) // Optional("array([JSON.dictionary([type: JSON.string(home), number: JSON.string(212 555-1234)]), JSON.dictionary([type: JSON.string(office), number: JSON.string(646 555-4567)]), JSON.dictionary([type: JSON.string(mobile), number: JSON.string(123 456-7890)])])")
print(decoded["isAlive"]?.boolean) // Optional(true)