I think it’s a far cry on the original author’s side to call the use of map().filter().map() an obsession with FP. Underscore.js simply provided super-useful functions over arrays and objects, which IMO makes for more legible code. Often times you can re-use filters with named functions. If you have multiple conditions for iteration exit, it’s clearer to chain filters than it is to have nested ifs or early exits in a for loop — it refactors better.

The real obsession with FP is the pervasive use of immutable state. But in apps, state must be effectively mutated, but now with a copy of the original immutable object. The FP obsession believes that this better than directly mutating the state each and every time.