fold-left-and-fold-right.js

function foldr (c, h) {
  return ([a, ...x]) => (a === undefined)
    ? c
    : h (a) (foldr(c, h)(x))
}

function foldl (c, h) {
  return ([a, ...x]) => (a === undefined)
    ? c
    : foldl (h(c)(a), h) (x)
}

const sum1 = foldr(0, x => y => { console.log(`${x} + ${y}`); return x + y })
const sum2 = foldl(0, x => y => { console.log(`${x} + ${y}`); return x + y })

const cat1 = foldr('0', x => y => `(${x} + ${y})`)
const cat2 = foldl('0', x => y => `(${x} + ${y})`)

console.log(
  cat1([1, 2, 3, 4, 5])
)
console.log(
  cat2([1, 2, 3, 4, 5])
)