fix(lib): Diff of array should consider both sides

src/lib/array-diff.js

@@ -2,18 +2,30 @@ function difference(a, b) {
   var hash = {}
   var diff = {}
 
-  b.forEach(function createHash(item) {
+  b.forEach(createHash(hash))
+  a.forEach(findDiff(hash, diff))
+
+  // return the other way round
+  hash = {}
+  a.forEach(createHash(hash))
+  b.forEach(findDiff(hash, diff))
+
+  return Object.keys(diff)
+
+}
+
+function createHash(hash) {
+  return function curried(item) {
     hash[item] = true
-  })
+  }
+}
 
-  a.forEach(function findDiff(item) {
+function findDiff(hash, diff) {
+  return function curried(item) {
     if (!hash[item] && !diff[item]) {
       diff[item] = true
     }
-  })
-
-  return Object.keys(diff)
-
+  }
 }
 
 module.exports = difference

test/lib/array-diff.js

@@ -5,15 +5,15 @@ describe('difference', function() {
   it('should return difference', function() {
     assert.deepEqual(
       difference(['a', 'b', 'c'], ['x', 'y', 'z']),
-      ['a', 'b', 'c']
+      ['a', 'b', 'c', 'x', 'y', 'z']
     )
     assert.deepEqual(
       difference(['a', 'b', 'c'], ['a', 'y', 'z']),
-      ['b', 'c']
+      ['b', 'c', 'y', 'z']
     )
     assert.deepEqual(
       difference(['a', 'b', 'c'], ['a', 'b', 'z']),
-      ['c']
+      ['c', 'z']
     )
 
     assert.deepEqual(
@@ -22,4 +22,3 @@ describe('difference', function() {
     )
   })
 })
-