Proof of concept that you can map multiple functions with only one iteration in JavaScript using iterators (like in Python). Run https://repl.it/Izou/0

imap.js

/*
 * imap lazily maps an interable.
 * @param fn            Map function
 * @param arrOrIterator Either an array or an iterator
 *
 * It works by always returning a new iterator so that
 * you can chain other imaps without loopin once more
 * over the same array!
 *
 * Since iterables are returned, to start the real mapping
 * you should do [...it] or use the function `list` I've made.
 */
const imap = (fn, arrOrIterator) => ({
  [Symbol.iterator]: () => {
    const it = arrOrIterator.next
        ? arrOrIterator 
        : arrOrIterator[Symbol.iterator]()
        
    return {
      next() {
        const {done, value} = it.next()
        return {done, value: done ? value : fn(value)}
      }
    }
  }
})

const double = x => 2 * x
const square = x => x * x
const list = it => [...it]

list(
  imap(square, 
    imap(double,[1, 2, 3, 4])))