// implement foldl (reduceLeft)
function foldl(arr, f, acc) {
    if (arr.length === 0) return acc 
    const [x, ...xs] = arr 
    return foldl(xs, f, f(acc, x))
}

# driver code
arr = [1,2,3,4,5]

parted = foldl(arr, (acc, x) => {
    const [a, b] = acc
    return (x % 2) 
        ? [[...a, x], [...b]]
        : [[...a], [...b, x]]
}, [[], []])

console.log(foldl(arr, (acc, x) => acc + x, 0))
console.log(foldl(arr, (acc, x) => acc * x, 1))
console.log(parted)