The Essential Guide to Javascript Sets
Tags: javascript, webdev, tutorial, typescript
Published: 2022-10-13 20:54:05

In a previous article, I covered [Javascript Maps](https://fjolt.com/article/javascript-map-data) - a data type very similar to objects, but with some key differences. Javascript has other data types too, and one you may not know much about is `Set`. A `Set` in Javascript is a unique list of values, similar to the [set data type in Python](https://fjolt.com/article/python-sets).

In this guide, we'll be looking at how sets work.

## What are Javascript Sets?
Javascript sets are a useful way of creating a unique list, which can be mutated, but contains **no** duplicates. A `Set` in Javascript is a great way to check if a value exists in a specified list of other values. Javascript sets are useful because they are typically faster than something like `Array.includes` - so they provide an efficient and optimised way of checking if a value exists in a set of other values. 

To create a new set, we use the `new Set()` constructor:
```javascript
let mySet = new Set();
```

## Basic Set Operations
It is one thing to initialize a new set, but how do we actually use it? `Set` has a number of operations, many of which will be familiar if you read my guide on maps. Let's take a look

### Adding elements to a Set
The `Set.add()` method is used to add elements to a set:
```javascript
let mySet = new Set();
mySet.add(5);
console.log(mySet); // Set(1) {5}
```

Sets can contain any data type - so you can add objects, numbers, strings, or anything else which you desire. If you try to add the same value twice, it will only appear in the set once:
```javascript
let mySet = new Set();
mySet.add(5);
mySet.add(5);
console.log(mySet); // Set(1) {5}
```

### Deleting elements from a Set
To delete a single element from a set in Javascript, you can use the `Set.delete()` method:
```javascript
let mySet = new Set();
mySet.add(5);
mySet.delete(5);
console.log(mySet); // Set(0) {}
```

You can also use the `Set.clear()` method to clear all items from a set:
```javascript
let mySet = new Set();
mySet.add(5);
mySet.add(10);
mySet.clear();
console.log(mySet); // Set(0) {}
```
### Deleting objects from a Set

Interestingly, objects are still created with separate references, so you can't delete an object from a set using the `Set.delete()` method. Trying to do so still results in the object item appearing in the set:
```javascript
let mySet = new Set();
mySet.add({o: 4});
mySet.delete({o: 4});
console.log(mySet); // Set(1) {{o: 4}}
```

Instead, if you want to delete an object from a set, you have to loop through the set using `forEach` or something similar. After finding the element you want to delete, simply delete it as normal:
```javascript
let mySet = new Set();
mySet.add({o: 4});
mySet.forEach((x) => { 
    if(x.o == 4) {
        mySet.delete(x)
    }
});
console.log(mySet); // Set(0) {}
```

### Set keys and entries
Sets in Javascript also have `Set.keys()` and `Set.entries()` methods. All sets are essentially a kind of array, so using `Set.keys()` simply returns an iterable set of keys from that set, as shown below:
```javascript
let mySet = new Set();
mySet.add(5);
mySet.add(10);
console.log(mySet.keys()); // SetIterator {5, 10}
```

`Set.entries()` is quite strange, since as I sort of mentioned, sets don't really have keys. In Javascript, typically an `entries()` method returns the key value pairs from a data structure in an array format, like `[key, value]`. Since sets don't have keys, `Set.entries()` just returns the same value for both key and value. The main reason this method exists is to provide consistency with the `Map` data type:
```javascript
let mySet = new Set();
mySet.add(5);
mySet.add(10);
let setEntries = mySet.entries();

for(let x of setEntries) {
    console.log(x);
    // Returns [ 5, 5 ], [ 10, 10 ]
}
```

## Checking Set Membership
The main use case for sets in Javascript is checking membership. This is slightly faster than using something like `Array.includes()`. To check set membership, you can use the `Set.has()` method. It returns `true`, if the set contains the item, and `false` if it does not:
```javascript
let mySet = new Set();
mySet.add(5);
mySet.add(10);
mySet.add(20);
console.log(mySet.has(10)); // true
```

Again, this will not work with objects exactly, since an object has a different reference when checked using `has`. As such, to check if an object is within a set, you must loop through it and check some value within the object:
```javascript
let mySet = new Set();
mySet.add({o: 4});
mySet.forEach((x) => { 
    if(x.o == 4) {
        // the object is in the set
    }
});
```

## Checking the size of a set
While arrays have `.length`, sets have `.size`. To check the size or length of a set, we can't use `.length`, like we normally would. Instead, the size of a set is found using `.size`:
```javascript
let mySet = new Set();
mySet.add(5);
mySet.add(10);
mySet.add(20);
console.log(mySet.size); // Returns 3
```

## Iterating over a Set
Sets themselves are able to be looped through with a simple for loop:
```javascript
let mySet = new Set();
mySet.add(5);
mySet.add(10);
mySet.add(20);
for(let x of mySet) {
    console.log(x); // Returns 5, 10, 20
}
```

However, sets also come with the `forEach()` method attached, so you can use that too. Typically `for` loops are faster, though:
```javascript
let mySet = new Set();
mySet.add(5);
mySet.add(10);
mySet.add(20);
mySet.forEach(() => {
    console.log(x); // Returns 5, 10, 20
}
```

As well as this, sets come with an iterator method attached too. You can access it like shown in the code below, and then use `next()` to iterate through each item in a set. You may not have seen this notation before, but many data structures and prototypes contain an iterator method. Here, we access it via `mySet[Symbol.iterator]()`.
```javascript
let mySet = new Set();
mySet.add(5);
mySet.add(10);
let setIterator = mySet[Symbol.iterator]();

console.log(setIterator.next()); // Returns { value: 5, done: false }
console.log(setIterator.next()); // Returns { value: 10, done: true }
```

## Conclusion

Sets are a super cool and little used data structure in Javascript, which give us a efficient way to check an item for its membership in another set of items. They are a little like arrays, but with a very specific purpose. If you've enjoyed this, you might also like my [Javascript Handbook](https://fjolt.com/series/learn-javascript). 

I hope you enjoyed this guide to sets, and have a great day.