Deep array flatten function

flattenArray.js

// SOLUTION:
// Turns out that the following is the most performant in terms of ops per seconds.
// It iterates on an array, staying within the "while loop" as long as
// the current item exists. Checking also whether using concat to the very same "arr" argument
// or push methods in a new stack array until no items are left within the "arr" argument
const flattenLoop = arr => {
  let stack = []
  let item

  while (item = arr.shift()) {
    if (Array.isArray(item)) {
      arr = item.concat(arr)
    } else {
      stack.push(item)
    }
  }

  return stack
}

// ALTERNATIVE APPROACH (less performant, but suggested as per MDN):
// This function is inspired by the following MDN "Alternative" section
// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/flatten
// and the draft proposal https://tc39.github.io/proposal-flatMap/#sec-Array.prototype.flatten
const flatten = (list) => {
  // Reducing everything to a single array
  return list.reduce((acc, val) => {
    // Checking whether return as param to concat straight val or go "flatten"
    // one level deeper for a nested array
    return acc.concat(Array.isArray(val) ? flatten(val) : val)
  }, [])
}