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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!
Conclusion
I hope this helped you understand the Set object and you now have a new tool in your JavaScript toolbelt!
References
Top comments (9)
Set and Map both are underused
Nice overview, this is the kind of "dev.to" article that I like :-)
For the toggle/binary state thing I tend to use plain JS objects, e.g.:
const isOpen = {};...
isOpen[menuItem] = true;
...
if (isOpen[menuItem]) ...
...
delete isOpen[menuItem];
But I agree that
Setis probably a better choice!Don't forget the super easy set-to array using
Array.from:-)Although I've used them some times, especially to remove duplicates in an array, never thought about the toggle example. I loved it!! Thanks
Cool, I never thought about that. Nide and useful example.
I will be considering Set in the future!
Wonderful use case，Can u post another blog about map?
Great post! I can't count the number of times the Set object has been extremely useful for challenges.
I've learn something new today. I haven't use Set yet in any of my projects but yes I agree Set might be a better option in some cases than array. Thanks
I use it all the time now! Glad you found it useful.