[spenuke]

I work on a similar team, with a mix of people who want to use functional styles and those who want something more imperative.

I lean toward the pro-functional style, but I also put a few strong constraints on when to use reduce. Namely, I would almost never use it in cases where I either want to (a) mutate something other than the accumulator or (b) include more than two branches of control flow in the reducer function.

This kind of goes for all the functional array methods. Many people are tempted to use them simply as alternate syntax for iterating through the array (i.e., a simple loop), where the loop body could contain several statements -- i.e., data mutation side effects, taking different execution paths based on certain conditions, etc. The functional style is way less clear in this sort of code.

The benefit of the functional style in the context of javascript array methods is when you can see at a glance what the shape of the resulting value is with respect to that of the source array. This is usually best done when the body of the reducer/mapper/filterer is a single expression.