What if we told you that one of the most useful operators in RxJS is one you probably have never heard of? In this talk, you'll meet the quiet workhorse called "groupBy." Don't let its understated nature fool you -- groupBy can change your life! The groupBy operator thrives in heavily interactive UIs, like when users rapidly favorite a bunch of items in a list. When combined with some of the other operators like concatMap, groupBy really shines. In this talk, you'll learn how to harness its power and take your reactive code to the next level. You will see how to use groupBy to give users an excellent experience, handling race conditions and back pressure with ease. You will also learn the *why* behind when to use groupBy and common pitfalls to avoid with it. Of course, you'll also leave with thorough sample code you can take back to work and start building from right away.