I like the "unpack" idea of using a function to name the parts of the tuple, it's elegant and fits well into JavaScript. I'm a bit less sure about exposing the "pack" method outside the constructor. That makes tuples mutable, which isn't what I normally want. Normally I want immutable collections of possibly mutable objects for my tuples. If I wanted a mutable collection I would use an array.

I absolutely hate the idea of "valueOf". Why would you ever in your life want to add together tuple values by default? It's a misfeature that is begging to cause bugs. A better feature would be some way of applying functions across tuples of the same size (map and fold type operations) to make it easy to build up correct comparison functions.