flattens nested list of arrays of integers

flattenArray.js

// VIEW ON JSFIDDLE -> https://jsfiddle.net/v5scgqao/3/
/*
flattens array without mutation
*/
function flattenArray(arr) {
  let arr2 = []
  function flatten(a) {
    for (let i = 0; i < a.length; i++) {
      if (Array.isArray(a[i]))
        flatten(a[i])
      else
        arr2.push(a[i])
    }
  }
  flatten(arr)
  return arr2
}