We used chunks of FP in a large frontend application. Where it came in most useful was in the data layer of the app -- the bits from calling out to external web

We used chunks of FP in a large frontend application. Where it came in most useful was in the data layer of the app -- the bits from calling out to external web services, adapting that data for the frontend views, and (because we used React) also rendering those views. Functions (including quite a lot of mapping and some reducing here and there) all the way down. We then ended up mixing in some WebGL for high performance parts, and that turned out to work best as more object-oriented code. Probably partly due to the library we used.