It's just like what @Unihedron said - and while reduce isn't the hardest thing to use, often times it's easy to forget that you can initialize the start value as anything (including an empty object) - the goal of the problem was to return a single object with the correct counts - reduce helps us accomplish that!

This one is short and sweet! Is it safe to say that "T" is being split off the string and then .join is being used to throw a "U" back in its place?

I like the way you did yours, I didn't realize I was allowed to change x and y to something else. Also, what does the 'use strict' bit mean?