A Set in Mathematics

If you think back to mathematics, a set is a collection of distinct items. For example, {2, 4, 5, 6} is a set because each number is unique and appears only once. However, {1, 1, 2, 4} is not a set because it contains duplicate entries (the 1 is in there more than once!).

In JavaScript, we can already represent something similar to a mathematical set using an array.

const nums = [2, 4, 5, 6];

However, arrays do not enforce items to be unique. If we try to add another 2 to nums, JavaScript won't complain and will add it without any issue.

nums.push(2);
console.log(nums);

[2, 4, 5, 6, 2]

…and now nums is no longer a set in the mathematical sense.

Sets

In ES6, there’s a new built-in object that behaves like a mathematical set and works similarly to an array. This new object is conveniently called a "Set". The biggest differences between a set and an array are:

• Sets are not indexed-based - you do not refer to items in a set based on their position in the set
• items in a Set can’t be accessed individually

Basically, a Set is an object that lets you store unique items. You can add items to a Set, remove items from a Set, and loop over a Set. These items can be either primitive values or objects.

How to Create a Set

There’s a couple of different ways to create a Set. The first way, is pretty straightforward:

const games = new Set();
console.log(games);

Set {}

This creates an empty Set games with no items.

If you want to create a Set from a list of values, you use an array:

const games = new Set(['Super Mario Bros.', 'Banjo-Kazooie', 'Mario Kart', 'Super Mario Bros.']);
console.log(games);

Set {'Super Mario Bros.', 'Banjo-Kazooie', 'Mario Kart'}

Notice the example above automatically removes the duplicate entry "Super Mario Bros." when the Set is created. Pretty neat!