Folding in JS

const foldl = f => z => ([x, ...xs]) =>
    x === undefined ? z
    : foldl (f) (f(z)(x)) (xs)

const foldr = f => z => ([x, ...xs]) =>
    x === undefined ? z
    : f (x) (foldr(f)(z)(xs))

// helper function
const cons    = x => ([...xs]) => [x,...xs]
const append  = ([...xs]) => ([...ys]) => [...xs, ...ys]
const apply   = f => x => f(x)
const flip    = f => a => b => f(b)(a)
const add = a => b => a + b
const mul = a => b => a * b
const and = a => b => a && b
const or  = a => b => a || b
const K   = a => _ => a;
const B   = g => f => x => g(f(x))


// In Terms Of Fold

// foldr_id - leaves a list intact, showing that folding a list means replacing all cons with `f` and the empty list ctor `[]` with an initial value, `z`
const foldr_id = foldr (cons)        ([])

const reverse  = foldl (flip (cons)) ([])
const flatten  = foldr (append)      ([])

const compose  = foldr (apply)
const pipe     = foldl (flip(apply))

const head     = foldr (K)       (undefined)
const last     = foldl (flip(K)) (undefined)

const sum      = foldr (add)  (0)
const product  = foldr (mul)  (1)

const all      = foldr (and)  (true)
const some     = foldr (or)   (false)

const map    = f => foldr (B(cons)(f)) ([])
const filter = p => foldr (x => xs => p(x) ? cons(x)(xs) : xs) ([])



// Check

let m = [1,2,3]
let n = [5,6,7]
let t = [[1,2,3], [5,6,7]]
let p = [true, true, false]

console.log(
    '\n',
    'foldr_id:'         , foldr_id  (m) , '\n' ,

    'reverse :'         , reverse   (m) , '\n' ,
    'flatten :'         , flatten   (t) , '\n' ,

    'sum     :'         , sum       (m) , '\n' ,
    'product :'         , product   (m) , '\n' ,

    'all     :'         , all       (p) , '\n' ,
    'some    :'         , some      (p) , '\n' ,

    'head    :'         , head      (m) , '\n' ,
    'last    :'         , last      (m) , '\n' ,

    'map     :'         , map    (add(10)) (m) , '\n' ,
    'filter  :'         , filter (a=>a>1)  (m) , '\n' ,

    'compose :'         , compose (5) ([x=>x*3, x=>x+5]) , '\n' ,
    'pipe    :'         , pipe    (5) ([x=>x*3, x=>x+5]) , '\n' ,
)