Add a function for the symmetric difference of two arrays

[eikes]

There was a previous pull request here, which explained why difference is not symmetric. The functionality hasn't changed though...

#277

[jashkenas]

Out of curiosity, do you have any real-world code that makes use of this function?

[jashkenas]

... apparently not.

[Aaronius]

Can we re-open this? Real-world example (names changed to keep it simple though):

I have an array of objects that contain metadata of widget A. I have an array of objects that contain metadata of widget B. I want to know which metadata objects are different between the two.

[Aaronius]

I should add that the commit from eikes only works with two arrays. I'd like it to work with an arbitrary number of arrays.

[michaelficarra]

_.symmetricDifference = function() {
  return _.reduce(arguments, function(first, second) {
    return _.union(_.difference(first, second), _.difference(second, first));
  });
}

[jashkenas]

Fixed in the above commit. While I think Michael's implementation is an exceptionally beautiful bit of code, if anyone feels like throwing together a faster implementation that's equally small, I'd be willing to take that patch.

[michaelficarra]

Yeah, I definitely don't like my implementation, but I figured I would adhere to the current convention of using other underscore functions in the definition of underscore functions. It'd probably be much more efficient if we removed all those function calls.

[eikes]

Neat!

[Aaronius]

Woohoo! Thanks!