flattenArray

/* Final Refactor */
// This is the final form of the function. I included the first iteration below to show my thought process as well as how the code was refactored
function flatten (arr) {
  if (!Array.isArray(arr)) return -1;

  return arr.reduce((p, c) => !Array.isArray(c) ? p.concat(c) : p.concat(flatten(c)), []);
}
/* End Final Refactor */

/* First Iteration - With Comments */
function flatten (arr) {
  // Established a check to determine if the array being passed in is actually an array - if not, return -1;
  if (!Array.isArray(arr)) return -1;
	
  // Reduce over the array
  return arr.reduce((p, c) => {
    // At each index, test if the item is not an array. If it is not, add it to the flattened list (p)
    if (!Array.isArray(c)) {
      return p.concat(c);
    } else {
      // If the index item is an array, recurse through applying the same logic on the subset array.
      return p.concat(flatten(c))
    }
  }, [])
}
/* End First Iteration */

Hey Jeff,
thanks for this submission, your code looks good!

Just one question: why does the flatten function return -1 in case the input is not an array?

Thanks!
Stefano