Flatten for underbar — recursive

flatten.js

// By the way — no idea why the function definition contains 'result' as a second argument;
// this is not present in the Underscore library.

  _.flatten = function(nestedArray, result) {
     var resultingArray = [];
     _.each(nestedArray, function(element, index, nestedArray){
       flattenHelper(element, resultingArray);
     });
     return resultingArray;
  };

  function flattenHelper(element, array){
    if (Array.isArray(element) === false){
      array.push(element);
    } else {
      _.each(element, function(item, i, element){
        flattenHelper(item, array);
      });
    }
  }