Demonstrate implementation of Array.flatten in javascript

flatten.js

function flattenRecursive(nested) {
  return nested.reduce((flat, item) => 
    flat.concat(Array.isArray(item) ? flatten(item) : item)
  , [])
}

function flattenIterative(nested) {
  let i = 0,
    flat = nested.slice() // copy array
  while (i < flat.length) {
    // continue untill current item is no longer an array
    if (Array.isArray(flat[i])) {
      // modify array by extracting current item into it
      flat.splice(i, 1, ...flat[i])
    } else {
      i++
    }
  }
  return flat
}

// default implementation is recursive
const flatten = (array, iterative) =>
  iterative ? flattenIterative(array) : flattenRecursive(array)

const test = [[[[1]],2,[3,4]],5,[]]
const expected = JSON.stringify([1,2,3,4,5])

console.assert(JSON.stringify(flatten(test)) === expected)
console.assert(JSON.stringify(flatten(test, true)) === expected)