Mastering JavaScript Functional Programming - Second Edition

Reducing an array to a value

Answer this question: how many times have you had to loop through an array, performing an operation (say, summing) to produce a single value (maybe the sum of all the array values) as a result? Probably many, many, many times. This kind of operation can usually be implemented functionally by applying reduce() and reduceRight(). Let's start with the former!

Time for some terminology! In usual FP parlance, we speak of folding operations: reduce() is foldl (for fold left) or just plain fold, and reduceRight() is correspondingly known as foldr. In category theory terms, both operations are catamorphisms: the reduction of all the values in a container down to a single result.

The inner workings of the reduce() function ...