old_contents: flattenArray.js
  
new_contents: /**
   * Flatten a nested array using Helper Method of Recursion
   *
   * Input:   Array of Integers and Arrays
   * Output:  Array of Integers
   *
   * Example: flatten([1, [2, 3, [4]], 5, [[6]]]) => [1, 2, 3, 4, 5, 6]
   **/
  
  function flatten(arr){
  
  }
  
subject: add desc