Use array destructuring, recursion, and the rest/spread operators to create a function 'double' that will return a new array with all values inside of it multiplied by two. Do not use any array helpers!

doublefunc.py

def fold(f, init, arr):
  return init if len(arr) < 1 else fold(f, f(init, arr[0]), arr[1:])

def map(f, xs):
  return fold(lambda acc, v: acc + [f(v)], [], xs)

def multiplyByTwo(x):
  def multiplySecond(y):
    return x * y
  return multiplySecond
print map(multiplyByTwo(2), [34,55,87])

doublewitfp.js

const fold = (f, init, xs) =>
    !xs.length ? init
            : fold(f, f(init, xs[0]), xs.slice(1));

const map = (f, xs) =>
    fold((acc, v) => [...acc, f(v)], [], xs);

const curriedMultiply = x => y => x * y;

const double = xs => map(curriedMultiply(2), xs);

console.log(double([1, 2, 3, 4]));

refactored.js

const fold = (f, init, xs) =>
    !xs.length ? init
             : fold(f, f(init, xs[0]), xs.slice(1));

const map = (f, xs) =>
    fold((acc, v) => [...acc, f(v)], [], xs);

console.log(map(x => x * 2, [1, 2, 3, 4]));