I'm not a functional purist, but I find that the functional 'principles' that I benefit the most from, in any context, are 1) massaging my data into the right shape with as few transforms as possible/practical/legible, 2) using pure functions whenever possible (in practice this often means accepting a few extra parameters in lieu of writing the code as a method; avoid 'this' in js, for example), and 3) starting with the data structures and seeing my app as, to a large degree, just a set of transformations on that (which usually is appropriate for the type of stuff I build).

I don't use partial application/currying that much, but more and more over time.

All in all I feel that doing these things have greatly improved my code and efficiency, and I'm kind of waiting for some downtime to get back into learning the more 'hardcore' functional languages.