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, title, challengeType, forumTopicId, dashedName

id	title	challengeType	forumTopicId	dashedName
587d7da9367417b2b2512b67	Add Elements to the End of an Array Using concat Instead of push	1	301226	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:

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.

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

Your code should not use the push method.

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

The first array should not change.

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

The second array should not change.

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

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

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

--seed--

--seed-contents--

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--

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