Flatten array

flatten.js

/**
 * Flattens native JS Array objects. 
 * Uses the native Array.isArray method, therefore array like objects (e.g Arguments object) need to be converted to Array
 * instances for this function to work on them (e.g ...arguments)
 * @param {Array} list 
 * @param {Array} newList - not required for the first call of function
 * @return {Array} newList
 */
function flatten(list, newList = []) {
  for (var i = 0, length = list.length; i < length; i++) {
    if (Array.isArray(list[i])) {
      flatten(list[i], newList);
    } else {
      newList.push(list[i]);
    }
  }
  
  return newList;
}