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What is a Set

A Set data structure allows to add data to a container.

ECMAScript 6 (also called ES2015) introduced the Set data structure to the JavaScript world, along with Map

A Set is a collection of objects or primitive types (strings, numbers or booleans), and you can think of it as a Map where values are used as map keys, with the map value always being a boolean true.

Initialize a Set

A Set is initialized by calling:

const s = new Set()

Add items to a Set

You can add items to the Set by using the add method:

s.add('one')
s.add('two')

A set only stores unique elements, so calling s.add('one') multiple times won’t add new items.

You can’t add multiple elements to a set at the same time. You need to call add() multiple times.

Check if an item is in the set

Once an element is in the set, we can check if the set contains it:

s.has('one') //true
s.has('three') //false

Delete an item from a Set by key

Use the delete() method:

s.delete('one')

Determine the number of items in a Set

Use the size property:

s.size

Delete all items from a Set

Use the clear() method:

s.clear()

Iterate the items in a Set

Use the keys() or values() methods - they are equivalent:

for (const k of s.keys()) {
  console.log(k)
}

for (const k of s.values()) {
  console.log(k)
}

The entries() method returns an iterator, which you can use like this:

const i = s.entries()
console.log(i.next())

calling i.next() will return each element as a { value, done = false } object until the iterator ends, at which point done is true.

You can also use the forEach() method on the set:

s.forEach(v => console.log(v))

or you can just use the set in a for..of loop:

for (const k of s) {
  console.log(k)
}

Initialize a Set with values

You can initialize a Set with a set of values:

const s = new Set([1, 2, 3, 4])

Convert to array

Convert the Set keys into an array

const a = [...s.keys()]

// or

const a = [...s.values()]

A WeakSet

A WeakSet is a special kind of Set.

In a Set, items are never garbage collected. A WeakSet instead lets all its items be freely garbage collected. Every key of a WeakSet is an object. When the reference to this object is lost, the value can be garbage collected.

Here are the main differences:

  1. you cannot iterate over the WeakSet
  2. you cannot clear all items from a WeakSet
  3. you cannot check its size

A WeakSet is generally used by framework-level code, and only exposes these methods:

  • add()
  • has()
  • delete()