One of my favorite helper functions is the groupBy function. There are versions of it in Lodash, Ramda, Underscore and elsewhere. Here I will wrap Underscore's version to do something very useful...
|I use an image not text so that you will type it out. Trust me. Take the time.|
Since every web app is basically JSON objects flowing in that you tinker with and then display, you can see how and why groupBy is so useful. Let's face it, the JSON you get from the server is never exactly how we want it for our UI app. It really shouldn't be, it should be at least somewhat generic so it can be used elsewhere.
In the battle to take what you get from your RESTful services and make it in to something that pleases third party plug-ins etc., groupBy and other functional helpers are priceless.