Array flattener

flatten.js

'use strict'

var input = [1, [2, 3, [4, [5], 6], 7, [8], 9], 10]

function flatten (array) {
  let result = []
  if (!Array.isArray(array) || array.length === 0) {
    return result
  }
  while (array.length > 0) {
    let next = array.shift()
    if (Array.isArray(next)) {
      let subResult = flatten(next)
      result = result.concat(subResult)
    } else {
      result.push(next)
    }
  }
  return result
}

console.log(flatten(input))