A tiny JavaScript library to create and manipulate multiple selections

Nice! What you could do is return this in most of the method in order to allow chaining. End result will be like selection.parent().next()

Only for navigating methods?

Yes, you can add these chaining abilities in only a few methods at first and then extend this principle to other ones if needed. As your API is quite similar to the one provided by jQuery, you can have a look at it to see which methods have this strategy.

SelectionList works in-place and on multiple selections, so it feels a bit strange to have this strategy.