---
id: a5de63ebea8dbee56860f4f2
title: Diff Two Arrays
isRequired: true
challengeType: 5
---

## Description
<section id='description'>
Compare two arrays and return a new array with any items only found in one of the two given arrays, but not both. In other words, return the symmetric difference of the two arrays.
Remember to use <a href='http://forum.freecodecamp.org/t/how-to-get-help-when-you-are-stuck/19514' target='_blank'>Read-Search-Ask</a> if you get stuck. Try to pair program. Write your own code.
<strong>Note</strong><br>You can return the array with its elements in any order.
</section>

## Instructions
<section id='instructions'>

</section>

## Tests
<section id='tests'>

```yml
tests:
  - text: <code>diffArray([1, 2, 3, 5], [1, 2, 3, 4, 5])</code> should return an array.
    testString: assert(typeof diffArray([1, 2, 3, 5], [1, 2, 3, 4, 5]) === "object", '<code>diffArray([1, 2, 3, 5], [1, 2, 3, 4, 5])</code> should return an array.');
  - text: <code>["diorite", "andesite", "grass", "dirt", "pink wool", "dead shrub"], ["diorite", "andesite", "grass", "dirt", "dead shrub"]</code> should return <code>["pink wool"]</code>.
    testString: assert.sameMembers(diffArray(["diorite", "andesite", "grass", "dirt", "pink wool", "dead shrub"], ["diorite", "andesite", "grass", "dirt", "dead shrub"]), ["pink wool"], '<code>["diorite", "andesite", "grass", "dirt", "pink wool", "dead shrub"], ["diorite", "andesite", "grass", "dirt", "dead shrub"]</code> should return <code>["pink wool"]</code>.');
  - text: <code>["diorite", "andesite", "grass", "dirt", "pink wool", "dead shrub"], ["diorite", "andesite", "grass", "dirt", "dead shrub"]</code> should return an array with one item.
    testString: assert(diffArray(["diorite", "andesite", "grass", "dirt", "pink wool", "dead shrub"], ["diorite", "andesite", "grass", "dirt", "dead shrub"]).length === 1, '<code>["diorite", "andesite", "grass", "dirt", "pink wool", "dead shrub"], ["diorite", "andesite", "grass", "dirt", "dead shrub"]</code> should return an array with one item.');
  - text: <code>["andesite", "grass", "dirt", "pink wool", "dead shrub"], ["diorite", "andesite", "grass", "dirt", "dead shrub"]</code> should return <code>["diorite", "pink wool"]</code>.
    testString: assert.sameMembers(diffArray(["andesite", "grass", "dirt", "pink wool", "dead shrub"], ["diorite", "andesite", "grass", "dirt", "dead shrub"]), ["diorite", "pink wool"], '<code>["andesite", "grass", "dirt", "pink wool", "dead shrub"], ["diorite", "andesite", "grass", "dirt", "dead shrub"]</code> should return <code>["diorite", "pink wool"]</code>.');
  - text: <code>["andesite", "grass", "dirt", "pink wool", "dead shrub"], ["diorite", "andesite", "grass", "dirt", "dead shrub"]</code> should return an array with two items.
    testString: assert(diffArray(["andesite", "grass", "dirt", "pink wool", "dead shrub"], ["diorite", "andesite", "grass", "dirt", "dead shrub"]).length === 2, '<code>["andesite", "grass", "dirt", "pink wool", "dead shrub"], ["diorite", "andesite", "grass", "dirt", "dead shrub"]</code> should return an array with two items.');
  - text: <code>["andesite", "grass", "dirt", "dead shrub"], ["andesite", "grass", "dirt", "dead shrub"]</code> should return <code>[]</code>.
    testString: assert.sameMembers(diffArray(["andesite", "grass", "dirt", "dead shrub"], ["andesite", "grass", "dirt", "dead shrub"]), [], '<code>["andesite", "grass", "dirt", "dead shrub"], ["andesite", "grass", "dirt", "dead shrub"]</code> should return <code>[]</code>.');
  - text: <code>["andesite", "grass", "dirt", "dead shrub"], ["andesite", "grass", "dirt", "dead shrub"]</code> should return an empty array.
    testString: assert(diffArray(["andesite", "grass", "dirt", "dead shrub"], ["andesite", "grass", "dirt", "dead shrub"]).length === 0, '<code>["andesite", "grass", "dirt", "dead shrub"], ["andesite", "grass", "dirt", "dead shrub"]</code> should return an empty array.');
  - text: <code>[1, 2, 3, 5], [1, 2, 3, 4, 5]</code> should return <code>[4]</code>.
    testString: assert.sameMembers(diffArray([1, 2, 3, 5], [1, 2, 3, 4, 5]), [4], '<code>[1, 2, 3, 5], [1, 2, 3, 4, 5]</code> should return <code>[4]</code>.');
  - text: <code>[1, 2, 3, 5], [1, 2, 3, 4, 5]</code> should return an array with one item.
    testString: assert(diffArray([1, 2, 3, 5], [1, 2, 3, 4, 5]).length  === 1, '<code>[1, 2, 3, 5], [1, 2, 3, 4, 5]</code> should return an array with one item.');
  - text: <code>[1, "calf", 3, "piglet"], [1, "calf", 3, 4]</code> should return <code>["piglet", 4]</code>.
    testString: assert.sameMembers(diffArray([1, "calf", 3, "piglet"], [1, "calf", 3, 4]), ["piglet", 4], '<code>[1, "calf", 3, "piglet"], [1, "calf", 3, 4]</code> should return <code>["piglet", 4]</code>.');
  - text: <code>[1, "calf", 3, "piglet"], [1, "calf", 3, 4]</code> should return an array with two items.
    testString: assert(diffArray([1, "calf", 3, "piglet"], [1, "calf", 3, 4]).length === 2, '<code>[1, "calf", 3, "piglet"], [1, "calf", 3, 4]</code> should return an array with two items.');
  - text: <code>[], ["snuffleupagus", "cookie monster", "elmo"]</code> should return <code>["snuffleupagus", "cookie monster", "elmo"]</code>.
    testString: assert.sameMembers(diffArray([], ["snuffleupagus", "cookie monster", "elmo"]), ["snuffleupagus", "cookie monster", "elmo"], '<code>[], ["snuffleupagus", "cookie monster", "elmo"]</code> should return <code>["snuffleupagus", "cookie monster", "elmo"]</code>.');
  - text: <code>[], ["snuffleupagus", "cookie monster", "elmo"]</code> should return an array with three items.
    testString: assert(diffArray([], ["snuffleupagus", "cookie monster", "elmo"]).length === 3, '<code>[], ["snuffleupagus", "cookie monster", "elmo"]</code> should return an array with three items.');
  - text: <code>[1, "calf", 3, "piglet"], [7, "filly"]</code> should return <code>[1, "calf", 3, "piglet", 7, "filly"]</code>.
    testString: assert.sameMembers(diffArray([1, "calf", 3, "piglet"], [7, "filly"]), [1, "calf", 3, "piglet", 7, "filly"], '<code>[1, "calf", 3, "piglet"], [7, "filly"]</code> should return <code>[1, "calf", 3, "piglet", 7, "filly"]</code>.');
  - text: <code>[1, "calf", 3, "piglet"], [7, "filly"]</code> should return an array with six items.
    testString: assert(diffArray([1, "calf", 3, "piglet"], [7, "filly"]).length === 6, '<code>[1, "calf", 3, "piglet"], [7, "filly"]</code> should return an array with six items.');

```

</section>

## Challenge Seed
<section id='challengeSeed'>

<div id='js-seed'>

```js
function diffArray(arr1, arr2) {
  var newArr = [];
  // Same, same; but different.
  return newArr;
}

diffArray([1, 2, 3, 5], [1, 2, 3, 4, 5]);
```

</div>



</section>

## Solution
<section id='solution'>


```js
function diffArray(arr1, arr2) {
  var newArr = [];
  var h1 = Object.create(null);
  arr1.forEach(function(e) {
    h1[e] = e;
  });

  var h2 = Object.create(null);
  arr2.forEach(function(e) {
    h2[e] = e;
  });

  Object.keys(h1).forEach(function(e) {
     if (!(e in h2)) newArr.push(h1[e]);
  });
  Object.keys(h2).forEach(function(e) {
     if (!(e in h1)) newArr.push(h2[e]);
  });
  // Same, same; but different.
  return newArr;
}
```

</section>