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

id, title, challengeType, forumTopicId

id	title	challengeType	forumTopicId
587d7b89367417b2b2512b48	Use the Spread Operator to Evaluate Arrays In-Place	1	301222

Description

ES6 introduces the spread operator, which allows us to expand arrays and other expressions in places where multiple parameters or elements are expected. The ES5 code below uses apply() to compute the maximum value in an array:

var arr = [6, 89, 3, 45];
var maximus = Math.max.apply(null, arr); // returns 89

We had to use Math.max.apply(null, arr) because Math.max(arr) returns NaN. Math.max() expects comma-separated arguments, but not an array. The spread operator makes this syntax much better to read and maintain.

const arr = [6, 89, 3, 45];
const maximus = Math.max(...arr); // returns 89

...arr returns an unpacked array. In other words, it spreads 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:

const spreaded = ...arr; // will throw a syntax error

Instructions

Copy all contents of arr1 into another array arr2 using the spread operator.

Tests

tests:
  - text: <code>arr2</code> should be correct copy of <code>arr1</code>.
    testString: assert(arr2.every((v, i) => v === arr1[i]) && arr2.length);
  - text: <code>...</code> spread operator should be used to duplicate <code>arr1</code>.
    testString: assert(code.match(/Array\(\s*\.\.\.arr1\s*\)|\[\s*\.\.\.arr1\s*\]/));
  - text: <code>arr2</code> should remain unchanged when <code>arr1</code> is changed.
    testString: assert((arr1, arr2) => {arr1.push('JUN'); return arr2.length < arr1.length});

Challenge Seed

const arr1 = ['JAN', 'FEB', 'MAR', 'APR', 'MAY'];
let arr2;

arr2 = [];  // Change this line

console.log(arr2);

Solution

const arr1 = ['JAN', 'FEB', 'MAR', 'APR', 'MAY'];
let arr2;

arr2 = [...arr1];