Issue jashkenas#309, adding symmetric difference as _.symDifference, …

…and making _.difference accept any number of arguments.

test/arrays.js

@@ -110,6 +110,14 @@ $(document).ready(function() {
   test("arrays: difference", function() {
     var result = _.difference([1, 2, 3], [2, 30, 40]);
     equals(result.join(' '), '1 3', 'takes the difference of two arrays');
+
+    var result = _.difference([1, 2, 3, 4], [2, 30, 40], [1, 11, 111]);
+    equals(result.join(' '), '3 4', 'takes the difference of three arrays');
+  });
+
+  test("arrays: symDifference", function() {
+    var result = _.symDifference([1, 2, 3], [2, 22, 222], [3, 33, 333], [222, 333, 444], [5]);
+    equals(result.join(' '), '1 22 33 444 5', 'takes the symmetric difference');
   });
 
   test('arrays: zip', function() {

underscore.js

@@ -402,10 +402,19 @@
     });
   };
 
-  // Take the difference between one array and another.
+  // Take the difference between one array and a number of other arrays.
   // Only the elements present in just the first array will remain.
-  _.difference = function(array, other) {
-    return _.filter(array, function(value){ return !_.include(other, value); });
+  _.difference = function(array) {
+    var rest = _.flatten(slice.call(arguments, 1));
+    return _.filter(array, function(value){ return !_.include(rest, value); });
+  };
+
+  // Take the symmetric difference between a list of arrays. Only the elements
+  // present in one of the input arrays will remain.
+  _.symDifference = function() {
+    return _.reduce(arguments, function(memo, array) {
+      return _.union(_.difference(memo, array), _.difference(array, memo));
+    });
   };
 
   // Zip together multiple lists into a single array -- elements that share