JSONSerialization extension to hone in on which values are invalid in complex Swift arrays or dictionaries.

JSONSerialization.swift

extension JSONSerialization {
    static func invalidJSONValue(data: Any) -> AnyObject? {
        if let elements = data as? [Any] {
            if !isValidJSONObject(data) {
                for element in elements {
                    if let value = invalidJSONValue(data: element) {
                        return value
                    }
                }

                for element in elements {
                    if !isValidJSONObject([element]) {
                        return element as AnyObject
                    }
                }

                return elements as AnyObject
            }
        } else if let elements = data as? [String: Any],
            let value = invalidJSONValue(data: Array(elements.values)) {
            return value as AnyObject
        }

        return nil
    }
}