code: function flatten(arr) {
    if (!isArray(arr)) {
      return arr;
    }
    var result = [];
    each(arr, function (item) {
      if (isArray(item)) {
        each(item, function (subItem) {
          result.push(subItem);
        });
      } else {
        result.push(item);
      }
    });
    return result;
  }
docs: Flattens `array` a single level deep.
  
  @param {Array} arr The array to flatten.
  @return {Array} Returns the new flattened array.
  @example
  
  _.flatten([1, [2, [3, [4]], 5]]);  // => [1, 2, [3, [4]], 5]