code: function avg (arr) {
      if (isArray(arr) && !(arr[0] instanceof Array)) {
          const totalSum = sum(arr);
          const len = arr.length || 1;
          return (Number.isNaN(totalSum) || totalSum instanceof InvalidAwareTypes) ?
                   InvalidAwareTypes.NULL : totalSum / len;
      }
      return InvalidAwareTypes.NULL;
  }
docs: Reducer function that returns the average of all the values.
  
  @public
  @param  {Array.<number>} arr - The input array.
  @return {number} Returns the mean value of the array.