Implementing folds via left and right

fold.js

'use strict';

let arr = [1,2,3,4,5];
let fn = (acc, x) => {
   console.log(`${acc}-${x}`); 
   return acc * x;
};
let seed = 1;

/*
foldl :: (a -> b -> a) -> a -> [b] -> a
foldl f a bs =
   foldr (\b g x -> g (f x b)) id bs a
*/
function reduceLeft(arr, fn, seed) {
  return arr.reduceRight((f, n) => x => f(fn(x, n)), x => x)(seed);
}

/*
foldr :: (b -> a -> a) -> a -> [b] -> a
foldr f a bs =
   foldl (\g b x -> g (f b x)) id bs a
*/
function reduceRight(arr, fn, seed) {
  return arr.reduce((f, n) => x => f(fn(x, n)), x => x)(seed);
}

reduceLeft(arr, fn, seed);
/*
1-1
1-2
2-3
6-4
24-5
> 120
*/

reduceRight(arr, fn, seed);
/*
1-5
5-4
20-3
60-2
120-1
> 120
*/