freeCodeCamp/curriculum/challenges/english/02-javascript-algorithms-and-data-structures/es6/use-the-spread-operator-to-evaluate-arrays-in-place.english.md

---
id: 587d7b89367417b2b2512b48
title: Use the Spread Operator to Evaluate Arrays In-Place
challengeType: 1
---

## Description
<section id='description'>
ES6 introduces the <dfn>spread operator</dfn>, which allows us to expand arrays and other expressions in places where multiple parameters or elements are expected.
The ES5 code below uses <code>apply()</code> to compute the maximum value in an array:
<blockquote>var arr = [6, 89, 3, 45];<br>var maximus = Math.max.apply(null, arr); // returns 89</blockquote>
We had to use <code>Math.max.apply(null, arr)</code> because <code>Math.max(arr)</code> returns <code>NaN</code>. <code>Math.max()</code> expects comma-separated arguments, but not an array.
The spread operator makes this syntax much better to read and maintain.
<blockquote>const arr = [6, 89, 3, 45];<br>const maximus = Math.max(...arr); // returns 89</blockquote>
<code>...arr</code> returns an unpacked array. In other words, it <em>spreads</em> the array.
However, the spread operator only works in-place, like in an argument to a function or in an array literal. The following code will not work:
<blockquote>const spreaded = ...arr; // will throw a syntax error</blockquote>
</section>

## Instructions
<section id='instructions'>
Copy all contents of <code>arr1</code> into another array <code>arr2</code> using the spread operator.
</section>

## Tests
<section id='tests'>

```yml
tests:
  - text: <code>arr2</code> is correct copy of <code>arr1</code>.
    testString: 'assert(arr2.every((v, i) => v === arr1[i]), "<code>arr2</code> is correct copy of <code>arr1</code>.");'
  - text: <code>...</code> spread operator was used to duplicate <code>arr1</code>.
    testString: 'getUserInput => assert(getUserInput("index").match(/\[\s*...arr1\s*\]/g),"<code>...</code> spread operator was used to duplicate <code>arr1</code>.");'
  - text: <code>arr2</code> remains unchanged when <code>arr1</code> is changed.
    testString: 'assert((arr1, arr2) => {arr1.push("JUN"); return arr2.length < arr1.length},"<code>arr2</code> remains unchanged when <code>arr1</code> is changed.");'

```

</section>

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

<div id='js-seed'>

```js
const arr1 = ['JAN', 'FEB', 'MAR', 'APR', 'MAY'];
let arr2;
(function() {
  "use strict";
  arr2 = []; // change this line
})();
console.log(arr2);
```

</div>



</section>

## Solution
<section id='solution'>

```js
// solution required
```
</section>