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Set is one of my favorite built-in object types in JavaScript. Today I'll introduce the Set object and discuss some of its use cases.

Video Tutorial

I made a video version of this tutorial! Check out this YouTube video if you prefer learning that way.

The Set Object

The Set object is a collection of values in which you can store unique primitive values or object references. Uniqueness is key here--no primitive value or object reference will can added multiple times.

How to Use Set

To use set, you create a new instance of it.

const mySet = new Set();

We now have an empty set. We can add the number 1 to this set by using the add method.

mySet.add(1);

How do we know that we've added 1? We can use the has method to check.

console.log(mySet.has(1));
// true

Let's add an object reference now and then check that we have that object in our Set.

const obj = { name: 'Daffodil' };
mySet.add(obj);
console.log(mySet.has(obj));
// true

Remember that object references are compared, not the object keys themselves. In other words:

console.log(mySet.has({ name: 'Daffodil' }));
// false

We can see how many elements are in the Set by using the size property.

console.log(mySet.size);
// 2

Next up, let's remove a value using the delete method.

mySet.delete(1);
console.log(mySet.has(1));
// false

Finally, we'll clear out the Set using the clear method.

mySet.clear();
console.log(mySet.size);
// 0

Iterating Over a Set

The easiest way to iterate over a Set is to use the forEach method.

new Set([1, 2, 3]).forEach(el => {
  console.log(el * 2);
});
// 2 4 6

Set objects also have entries, keys, and values methods, which each returns Iterators. Those are a bit outside the scope of this tutorial!

Using Sets in the Wild

I find the Set object to be really great for keeping track of a binary state associated with an object. A great example is an accordion menu: each item in the menu will either be open or closed. We can create a Set called isOpen that tracks the open status of an accordion item and a toggle function that toggles the open status:

const isOpen = new Set();

function toggle(menuItem) {
  if (isOpen.has(menuItem)) {
    isOpen.delete(menuItem);
  } else {
    isOpen.add(menuItem);
  }
}

A Note of Efficiency

You might be thinking that the Set object seems awfully similar to arrays. There is, however, a big difference that may have performance ramifications in your application. The Set object is required to be implemented using hash tables (or methods with hash table-like efficiency) [1].

When you store something in an array, you might have to traverse the entire array to find the item. With a Set, however, the lookup is instantaneous. Practically speaking, the performance will be negligable for most cases, but good to remember if you find yourself having to track large numbers of items!