freeCodeCamp/curriculum/challenges/english/02-javascript-algorithms-and-data-structures/functional-programming/add-elements-to-the-end-of-an-array-using-concat-instead-of-push.md

---
id: 587d7da9367417b2b2512b67
title: Add Elements to the End of an Array Using concat Instead of push
challengeType: 1
forumTopicId: 301226
dashedName: add-elements-to-the-end-of-an-array-using-concat-instead-of-push
---

# --description--

Functional programming is all about creating and using non-mutating functions.

The last challenge introduced the `concat` method as a way to combine arrays into a new one without mutating the original arrays. Compare `concat` to the `push` method. `push` adds an item to the end of the same array it is called on, which mutates that array. Here's an example:

```js
const arr = [1, 2, 3];
arr.push([4, 5, 6]);
```

`arr` would have a modified value of `[1, 2, 3, [4, 5, 6]]`, which is not the functional programming way.

`concat` offers a way to add new items to the end of an array without any mutating side effects.

# --instructions--

Change the `nonMutatingPush` function so it uses `concat` to add `newItem` to the end of `original` instead of `push`. The function should return an array.

# --hints--

Your code should use the `concat` method.

```js
assert(code.match(/\.concat/g));
```

Your code should not use the `push` method.

```js
assert(!code.match(/\.?[\s\S]*?push/g));
```

The `first` array should not change.

```js
assert(JSON.stringify(first) === JSON.stringify([1, 2, 3]));
```

The `second` array should not change.

```js
assert(JSON.stringify(second) === JSON.stringify([4, 5]));
```

`nonMutatingPush([1, 2, 3], [4, 5])` should return `[1, 2, 3, 4, 5]`.

```js
assert(
  JSON.stringify(nonMutatingPush([1, 2, 3], [4, 5])) ===
    JSON.stringify([1, 2, 3, 4, 5])
);
```

# --seed--

## --seed-contents--

```js
function nonMutatingPush(original, newItem) {
  // Only change code below this line
  return original.push(newItem);

  // Only change code above this line
}

const first = [1, 2, 3];
const second = [4, 5];
nonMutatingPush(first, second);
```

# --solutions--

```js
function nonMutatingPush(original, newItem) {
  return original.concat(newItem);
}
const first = [1, 2, 3];
const second = [4, 5];
```