Change:
], "tests": [ "assert(typeof(diff([1, 2, 3, 5], [1, 2, 3, 4, 5])) === "object", 'message: <code>diff()</code> should return an array.');", <del> "assert.deepEqual(diff(["diorite", "andesite", "grass", "dirt", "pink wool", "dead shrub"], ["diorite", "andesite", "grass", "dirt", "dead shrub"]), ["pink wool"], 'message: <code>["diorite", "andesite", "grass", "dirt", "pink wool", "dead shrub"], ["diorite", "andesite", "grass", "dirt", "dead shrub"]</code> should return <code>["pink wool"]</code>.');", "assert.includeMembers(diff(["andesite", "grass", "dirt", "pink wool", "dead shrub"], ["diorite", "andesite", "grass", "dirt", "dead shrub"]), ["diorite", "pink wool"], 'message: <code>["andesite", "grass", "dirt", "pink wool", "dead shrub"], ["diorite", "andesite", "grass", "dirt", "dead shrub"]</code> should return <code>["diorite", "pink wool"]</code>.');", "assert.deepEqual(diff(["andesite", "grass", "dirt", "dead shrub"], ["andesite", "grass", "dirt", "dead shrub"]), [], 'message: <code>["andesite", "grass", "dirt", "dead shrub"], ["andesite", "grass", "dirt", "dead shrub"]</code> should return <code>[]</code>.');", "assert.deepEqual(diff([1, 2, 3, 5], [1, 2, 3, 4, 5]), [4], 'message: <code>[1, 2, 3, 5], [1, 2, 3, 4, 5]</code> should return <code>[4]</code>.');", "assert.includeMembers(diff([1, "calf", 3, "piglet"], [1, "calf", 3, 4]), ["piglet", 4], 'message: <code>[1, "calf", 3, "piglet"], [1, "calf", 3, 4]</code> should return <code>["piglet", 4]</code>.');", "assert.deepEqual(diff([], ["snuffleupagus", "cookie monster", "elmo"]), ["snuffleupagus", "cookie monster", "elmo"], 'message: <code>[], ["snuffleupagus", "cookie monster", "elmo"]</code> should return <code>["snuffleupagus", "cookie monster", "elmo"]</code>.');", "assert.includeMembers(diff([1, "calf", 3, "piglet"], [7, "filly"]), [1, "calf", 3, "piglet", 7, "filly"], 'message: <code>[1, "calf", 3, "piglet"], [7, "filly"]</code> should return <code>[1, "calf", 3, "piglet", 7, "filly"]</code>.');" </del> <ins> "assert.sameMembers(diff(["diorite", "andesite", "grass", "dirt", "pink wool", "dead shrub"], ["diorite", "andesite", "grass", "dirt", "dead shrub"]), ["pink wool"], 'message: <code>["diorite", "andesite", "grass", "dirt", "pink wool", "dead shrub"], ["diorite", "andesite", "grass", "dirt", "dead shrub"]</code> should return <code>["pink wool"]</code>.');", "assert.sameMembers(diff(["andesite", "grass", "dirt", "pink wool", "dead shrub"], ["diorite", "andesite", "grass", "dirt", "dead shrub"]), ["diorite", "pink wool"], 'message: <code>["andesite", "grass", "dirt", "pink wool", "dead shrub"], ["diorite", "andesite", "grass", "dirt", "dead shrub"]</code> should return <code>["diorite", "pink wool"]</code>.');", "assert.sameMembers(diff(["andesite", "grass", "dirt", "dead shrub"], ["andesite", "grass", "dirt", "dead shrub"]), [], 'message: <code>["andesite", "grass", "dirt", "dead shrub"], ["andesite", "grass", "dirt", "dead shrub"]</code> should return <code>[]</code>.');", "assert.sameMembers(diff([1, 2, 3, 5], [1, 2, 3, 4, 5]), [4], 'message: <code>[1, 2, 3, 5], [1, 2, 3, 4, 5]</code> should return <code>[4]</code>.');", "assert.sameMembers(diff([1, "calf", 3, "piglet"], [1, "calf", 3, 4]), ["piglet", 4], 'message: <code>[1, "calf", 3, "piglet"], [1, "calf", 3, 4]</code> should return <code>["piglet", 4]</code>.');", "assert.sameMembers(diff([], ["snuffleupagus", "cookie monster", "elmo"]), ["snuffleupagus", "cookie monster", "elmo"], 'message: <code>[], ["snuffleupagus", "cookie monster", "elmo"]</code> should return <code>["snuffleupagus", "cookie monster", "elmo"]</code>.');", "assert.sameMembers(diff([1, "calf", 3, "piglet"], [7, "filly"]), [1, "calf", 3, "piglet", 7, "filly"], 'message: <code>[1, "calf", 3, "piglet"], [7, "filly"]</code> should return <code>[1, "calf", 3, "piglet", 7, "filly"]</code>.');" </ins> ], "MDNlinks": [ "Comparison Operators",

Discussion:
Challenge has an issue. User Agent is: <codeMozilla/5.0 (Windows NT 6.1; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/48.0.2547.0 Safari/537.36</code. My following code successfully satisfies the criteria and all the tests, except one due to an inconsistency. ! The returned array will be sorted due to my method. The last test has no issue with my output being in sorted order, yet expects the output to retain the same order, as my output is not accepted. This is inconsistent, and it should also accept the answer regardless of its order. To bypass this I would have to implement a check to see if one of the given arrays is empty and if so, return the other as is, which is not part of the exercise's criteria.
Thanks guys!

The Bonfire says "Compare two arrays and return a new array with any items not found in both of the original arrays.". If you can't find an item in both arrays then the result will be infinite. The wording may be less confusing if it says: "Compare two arrays and return a new array with any items in the second array that are not found in the first."
Working on this!
"Compare two arrays and return a new array with any items in the second array that are not found in the first or any items in the first array that are not found in the second." i.e. "Return any items which doesn't exist in both the arrays."