Swift extensions for retrieving values from a JSON response

GetJSONValue.swift

extension Dictionary where Key: StringLiteralConvertible, Value:AnyObject {
    subscript(keysAndIndexes: AnyObject ...) -> AnyObject? {
        guard let jsonObject = self as? AnyObject else { fatalError("\(self) is not a json dictionary") }
        return _extractValueFrom(keysAndIndexes, from: jsonObject)
    }
}

extension Array where Element:AnyObject {
    subscript(keysAndIndexes: AnyObject ...) -> AnyObject? {
        return _extractValueFrom(keysAndIndexes, from: self)
    }
}

private func _extractValueFrom(keysAndIndexes: [AnyObject], from: AnyObject) -> AnyObject? {
    var previousValue: AnyObject? = from
    
    for keyOrIndex in keysAndIndexes {
        if let key = keyOrIndex as? String, dict = previousValue as? Dictionary<String, AnyObject> {
            // a key (String) was passed and previous value is a dictionary
            previousValue = dict[key]
        } else if let index = keyOrIndex as? Int, array = previousValue as? Array<AnyObject> {
            // an index (Int) was passed and previous value is an array
            previousValue = array[index]
        } else {
            return nil
        }
    }
    return previousValue
}