WeakSet

Definition

A WeakSet is a collection of unique objects. Like WeakMap, WeakSet allows garbage collection of its elements when they are no longer referenced elsewhere. WeakSets are useful for tracking object membership without preventing garbage collection.

const weakSet = new WeakSet()
weakSet instanceof WeakSet  // true

Creating weaksets

// Empty WeakSet
const weakSet = new WeakSet()

// From array of objects
const obj1 = {}
const obj2 = {}
const weakSet2 = new WeakSet([obj1, obj2])

Weakset methods

Adding and checking

const weakSet = new WeakSet()
const obj = {}

// add() - adds an object
weakSet.add(obj)

// has() - checks if object exists
weakSet.has(obj)  // true
weakSet.has({})    // false (different object)

// delete() - removes an object
weakSet.delete(obj)  // true
weakSet.has(obj)    // false

Key differences from set

Feature	WeakSet	Set
Value types	Objects only	Any type
Iteration	Not iterable	Iterable
Size	No size property	Has size property
Garbage collection	Values can be GC'd	Values prevent GC
Methods	add, has, delete	Full API

Use cases

Tracking object membership

const processed = new WeakSet()

function processObject(obj) {
  if (processed.has(obj)) {
    return 'Already processed'
  }
  
  // Process object
  processed.add(obj)
  return 'Processed'
}

Preventing circular references

const visited = new WeakSet()

function traverse(obj) {
  if (visited.has(obj)) {
    return  // Already visited, prevent infinite loop
  }
  
  visited.add(obj)
  // Continue traversal
}

Tagging objects

const tagged = new WeakSet()

function tag(obj) {
  tagged.add(obj)
}

function isTagged(obj) {
  return tagged.has(obj)
}

const obj = {}
tag(obj)
isTagged(obj)  // true

Why weakset?

WeakSets allow garbage collection of their elements:

let obj = { id: 1 }
const weakSet = new WeakSet()
weakSet.add(obj)

obj = null  // Object can now be garbage collected
// WeakSet entry is automatically removed

This is different from Set:

let obj = { id: 1 }
const set = new Set()
set.add(obj)

obj = null  // Object cannot be garbage collected
// Set still holds reference to obj

Limitations

1. Not iterable: Cannot use for...of, forEach(), or get size.

2. Objects only: Values must be objects, not primitives.

3. No size property: Cannot check how many objects are in the WeakSet.

4. Limited API: Only add(), has(), and delete() methods.

Best practices

1. Use for object tracking: Track which objects have been processed or visited.

2. Use for tagging: Mark objects with metadata without modifying them.

3. Let garbage collection work: Don't hold references to objects elsewhere if you want automatic cleanup.

4. Don't use for iteration: If you need to iterate, use Set instead.

5. Understand garbage collection: WeakSets are designed to work with GC—use them when this behavior is desired.