Set

Definition

A Set is a collection of unique values. Each value can occur only once in a Set. Sets maintain insertion order and provide efficient membership testing.

> const set = new Set()
undefined

> set instanceof Set
true

Using sets

// Empty set
const set = new Set()

// From array
const set2 = new Set([1, 2, 3, 3, 2, 1])
// Set { 1, 2, 3 } (duplicates removed)

// From string
const set3 = new Set('hello')
// Set { 'h', 'e', 'l', 'o' } (duplicates removed)

Operations on sets

Set methods

Adding and Checking

const set = new Set()

// add() - adds a value
set.add(1)
set.add(2)
set.add(1)  // Ignored (duplicate)

// has() - checks if value exists
set.has(1)  // true
set.has(3)  // false

Size

const set = new Set([1, 2, 3])
set.size  // 3

Deleting

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

// delete() - removes a value
set.delete(2)  // true (value existed)
set.delete(4)  // false (value didn't exist)

// clear() - removes all values
set.clear()
set.size  // 0

Iteration

for...of Loop

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

for (const value of set) {
  console.log(value)
}
// 1
// 2
// 3

forEach()

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

set.forEach(value => {
  console.log(value)
})

values(), keys(), entries()

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

Array.from(set.values())  // [1, 2, 3]
Array.from(set.keys())    // [1, 2, 3] (same as values)
Array.from(set.entries()) // [[1, 1], [2, 2], [3, 3]]

Set operations

Union

const set1 = new Set([1, 2, 3])
const set2 = new Set([3, 4, 5])

const union = new Set([...set1, ...set2])
// Set { 1, 2, 3, 4, 5 }

Intersection

const set1 = new Set([1, 2, 3])
const set2 = new Set([2, 3, 4])

const intersection = new Set(
  [...set1].filter(x => set2.has(x))
)
// Set { 2, 3 }

Difference

const set1 = new Set([1, 2, 3])
const set2 = new Set([2, 3, 4])

const difference = new Set(
  [...set1].filter(x => !set2.has(x))
)
// Set { 1 }

Use Cases

Removing Duplicates

const array = [1, 2, 2, 3, 3, 3]
const unique = [...new Set(array)]
// [1, 2, 3]

Membership Testing

const allowedUsers = new Set(['alice', 'bob', 'charlie'])

function isAllowed(username) {
  return allowedUsers.has(username)
}

Tracking Visited Items

const visited = new Set()

function visit(item) {
  if (visited.has(item)) {
    return 'Already visited'
  }
  visited.add(item)
  return 'First visit'
}

Set vs Array

Feature	Set	Array
Uniqueness	Enforced automatically	Manual checking
Lookup	O(1) average	O(n)
Order	Insertion order	Index-based
Duplicates	Not allowed	Allowed

Best Practices

1. Use Set for uniqueness: When you need to ensure values are unique.

2. Use Set for membership testing: Faster than array.includes() for large collections.

3. Convert to array when needed: Use spread operator [...set] to convert back to array.

4. Use Set for deduplication: Simple way to remove duplicates from arrays.

5. Consider WeakSet: For object-only collections that allow garbage collection.