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.english.md

id, title, challengeType

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

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:

var arr = [1, 2, 3];
arr.push([4, 5, 6]);
// arr is changed to [1, 2, 3, [4, 5, 6]]
// 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.

Tests

tests:
  - text: Your code should use the <code>concat</code> method.
    testString: assert(code.match(/\.concat/g));
  - text: Your code should not use the <code>push</code> method.
    testString: assert(!code.match(/\.push/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>nonMutatingPush([1, 2, 3], [4, 5])</code> should return <code>[1, 2, 3, 4, 5]</code>.
    testString: assert(JSON.stringify(nonMutatingPush([1, 2, 3], [4, 5])) === JSON.stringify([1, 2, 3, 4, 5]));

Challenge Seed

function nonMutatingPush(original, newItem) {
  // Add your code below this line
  return original.push(newItem);

  // Add your code above this line
}
var first = [1, 2, 3];
var second = [4, 5];
nonMutatingPush(first, second);

Solution

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