freeCodeCamp/curriculum/challenges/english/02-javascript-algorithms-and-data-structures/functional-programming/combine-two-arrays-using-the-concat-method.english.md

---
id: 587d7da9367417b2b2512b66
title: Combine Two Arrays Using the concat Method
challengeType: 1
isHidden: false
forumTopicId: 301229
---

## Description
<section id='description'>
<dfn>Concatenation</dfn> means to join items end to end. JavaScript offers the <code>concat</code> method for both strings and arrays that work in the same way. For arrays, the method is called on one, then another array is provided as the argument to <code>concat</code>, which is added to the end of the first array. It returns a new array and does not mutate either of the original arrays. Here's an example:

```js
[1, 2, 3].concat([4, 5, 6]);
// Returns a new array [1, 2, 3, 4, 5, 6]
```

</section>

## Instructions
<section id='instructions'>
Use the <code>concat</code> method in the <code>nonMutatingConcat</code> function to concatenate <code>attach</code> to the end of <code>original</code>. The function should return the concatenated array.
</section>

## Tests
<section id='tests'>

```yml
tests:
  - text: Your code should use the <code>concat</code> method.
    testString: assert(code.match(/\.concat/g));
  - text: The <code>first</code> array should not change.
    testString: assert(JSON.stringify(first) === JSON.stringify([1, 2, 3]));
  - text: The <code>second</code> array should not change.
    testString: assert(JSON.stringify(second) === JSON.stringify([4, 5]));
  - text: <code>nonMutatingConcat([1, 2, 3], [4, 5])</code> should return <code>[1, 2, 3, 4, 5]</code>.
    testString: assert(JSON.stringify(nonMutatingConcat([1, 2, 3], [4, 5])) === JSON.stringify([1, 2, 3, 4, 5]));

```

</section>

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

<div id='js-seed'>

```js
function nonMutatingConcat(original, attach) {
  // Only change code below this line


  // Only change code above this line
}
var first = [1, 2, 3];
var second = [4, 5];
nonMutatingConcat(first, second);
```

</div>



</section>

## Solution
<section id='solution'>

```js
function nonMutatingConcat(original, attach) {
  // Only change code below this line
  return original.concat(attach);
  // Only change code above this line
}
var first = [1, 2, 3];
var second = [4, 5];
nonMutatingConcat(first, second);
```

</section>