I don't think it helps a lot to suggest one specific technology, because nobody actually knows the future (what that next big thing will be).
Stay agile, aware, and receptive. Learn how to learn better. Don't wed yourself off for life to one technology, and don't grow stubborn. Be prepared to turn the ship, quickly, when it comes time. Learn JS today? Sure, I'd argue however that it's far more important that you're ready to alter your priorities when that Thing comes along and smashes whatever you think is important today.
Stay agile, aware, and receptive. Learn how to learn better. Don't wed yourself off for life to one technology, and don't grow stubborn. Be prepared to turn the ship, quickly, when it comes time. Learn JS today? Sure, I'd argue however that it's far more important that you're ready to alter your priorities when that Thing comes along and smashes whatever you think is important today.