Learn JavaScript Sets (Simple Yet Powerful Built-In Objects)

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.

The Set Object

The Set object is an object in which you can store unique primitive values or object references. Uniqueness is key here--no primitive value or object reference can be 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!

Conclusion

I hope this helped you understand the Set object and you now have a new tool in your JavaScript toolbelt!

---

References

1. Set Object Specification

Comments

[Oğuzhan Olguncu]

I actually run some benchmarks, and the results were fascinating. Here is the scenario: I've created an array and a set with full of True values and with only one False at the end.

By the way, if you increase the number of values in an array it will take even longer because it has O(n) complexity. In essence, it looks up every item in the array one by one. Whereas set has O(1) complexity meaning it takes the same amount of time even though the number grows.

[Ilya Sher]

Not a fair comparison on several dimensions.

• Set should have only unique values. If you modify populateArray to only store unique elements - that would be an improvement.
• Set should typically be compared to Object

[Oğuzhan Olguncu]

Yes, you are right. But, even if we've stored unique values inside populateArray such as alphabet(a-z) it would still have taken more time than Set. Because it still has to iterate it over with O(n) complexity. What I was trying to show is a mere demonstration of execution of speed

[Nick Scialli (he/him)]

This is awesome, thank you! This should probably be its own post

[Kenneth]

is there a difference if you use Array.includes?

[nqthqn]

Computing the difference or intersection between sets can also be helpful. I used this the other day to determine what remaining items I had to find in a computation.

// intersect can be simulated via 
let intersection = new Set([...set1].filter(x => set2.has(x)))

// difference can be simulated via
let difference = new Set([...set1].filter(x => !set2.has(x)))

Compliments of MDN.

[Matthew Bramer]

Thanks for the note about Sets using hash table lookups and the link to the spec. Didn't know that about Set.