First time I've heard of RunKit. I've used Jupyter in the past to do a similar thing (minus the API stuff which is very cool). What does RunKit provide that Jupyter doesn't?
You might have not heard of us because we used to be named Tonic ;) There are a number of different factors that differentiate us from Jupyter, but we have of course taken a ton of inspiration from them. The data science crowd including iPython, Mathematica, etc have pioneered a development style that I hope see make its way into "normal" programming. One of the best resources is a pretty in-depth blog post we wrote about what's going on under the hood here: http://blog.runkit.com/2015/09/10/time-traveling-in-node.js-...
But, to give a quick overview:
1. One of the original driving goals when we started working on RunKit was to completely eliminate friction in using other people's code. So what we did was install every permutation of every version of every package for node. You can even require two versions at once and compare them: https://runkit.com/tolmasky/api-diff-example . This has lead to many interesting uses that aren't possible with other notebooks. For example, many people use us for filing bugs, because its kind of the dream: you're not filing a bug, you're filing a computer and environment with an undeniable reproduction, that the reviewer can easily test (what happens if I change node version? what happens if I change module version, etc etc).
2. As listed in the blog post above, we've spent a tremendous amount of time on the problem of determinism, in an attempt to "unify" the REPL and file environments. Basically, RunKit notebooks read top to bottom, just like a file, and we guarantee that if you take the contents of a notebooks and run it in normal node, it should behave exactly the same. For example, no asynchronous events should be first in-between consecutive cells - this is really important in asynchronous programming. To be able to deliver on this guarantee we invested heavily on "time traveling" -- when you edit a cell in the past, the ENTIRE virtual machine is rewound, files you had deleted are restored, etc (again, more info in the blog post if you find this interesting).
Happy to answer any other questions, apologies for the rambling, haven't gotten much sleep!