Convert JSON string array to NSArray in swift

You have to call JSONSerialization.jsonObject twice. First to deserialize the root object and then to deserialize the JSON string for key abc.

func fromJSON(string: String) throws -> [[String: Any]] {
    let data = Data(string.utf8)
    guard let rootObject = try JSONSerialization.jsonObject(with: data) as? [String:String],
        let innerJSON = rootObject["abc"]  else {
            throw NSError(domain: NSCocoaErrorDomain, code: 1, userInfo: [NSLocalizedDescriptionKey: "Invalid JSON"])
    }
    let innerData = Data(innerJSON.utf8)
    guard let innerObject = try JSONSerialization.jsonObject(with: innerData) as? [[String:Any]] else {
        throw NSError(domain: NSCocoaErrorDomain, code: 1, userInfo: [NSLocalizedDescriptionKey: "Invalid JSON"])
    }
    return innerObject
}

Another more comfortable approach is to decode the string with Decodable

let jsonString = """
{"abc":"[{\\"ischeck\\":true,\\"type\\":\\"Some type\\"},{\\"ischeck\\":false,\\"type\\":\\"other type\\"}]"}
"""

struct Root : Decodable {
    let abc : [Item]

    private enum CodingKeys : String, CodingKey { case abc }

    init(from decoder : Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        let abcString = try container.decode(String.self, forKey: .abc)
        abc = try JSONDecoder().decode([Item].self, from: Data(abcString.utf8))
    }
}

struct Item : Decodable {
    let ischeck : Bool
    let type : String
}

do {
   let result = try JSONDecoder().decode(Root.self, from: Data(jsonString.utf8))
   print(result.abc) 
} catch {
    print(error)
}