The Care and Feeding of the JavaScript Set OBject

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.

What is 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 can be added multiple times.

“That’s great,” you say, “but how can I use it?” Well, I’m glad you asked!

To use a set, you have to, as you might imagine, create a new instance of it.

const mySet = new Set();

So now we have an empty set. We can do fun stuff like add the number 1 to this set by using the add method.

myset.add(1);

“Do you have any proof that it added it,” you ask? You’re not very trusting, so here, I’ll show you. We use the has method to check if 1 is really in the set.

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

Ok, so now you have some proof that this really worked. Let’s get a little crazier and add an object reference now. Then, we will check that we have that object in our Set.

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

Pretty simple, right? One thing to remember, is that object references are compared, not the object keys themselves. For example:

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

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

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

Now, let’s try deleting a value from the Set by using the delete method.

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

Ok, one more thing, let’s empty the Set by using the clear method.

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

Let’s try iterating over a Set

There is a super easy way to iterate over a Set and that’s by using the forEach method.

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

To review, Set objects have entries, keys, and values methods. Each of those returns Iterators. Take a look at the excellent MDN resource for more information about Iterators.

How do I use these in the real world?

The Set object is an excellent resource for keeping track of a binary state associated with an object. For example, an accordion menu. As you are aware, each menu item will have a state of either open or closed. We would create an object (or Set) that would keep track of the open status of each accordion item and a toggle function that switches the open status.

const isOpen = new Set();

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

Just how efficient is this Set object anyway?

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

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 negligible for most cases, but good to remember if you find yourself having to track large numbers of items!