Set vs WeakSet

Set

A Set is a collection of unique values.

Features

• Stores primitives and objects
• Iterable (for...of, forEach)
• Has a size property
• Prevents stored objects from being garbage collected

Methods

add()
delete()
has()
clear()

Properties

size

Iteration

keys()
values()
entries()
forEach()
for...of

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WeakSet

A WeakSet is a collection of unique objects only.

Features

• Stores objects only (arrays, functions, objects)
• Cannot store primitives
• Holds weak references
• Objects are automatically removed when garbage collected
• Not iterable
• No size property

Methods

add()
delete()
has()

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Weak References

A WeakSet does not prevent an object from being garbage collected.

Set

let obj = { id: 1 };

const set = new Set();
set.add(obj);

obj = null;

Set ─────► Object

• Strong reference
• Object remains in memory

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WeakSet

let obj = { id: 1 };

const weakSet = new WeakSet();
weakSet.add(obj);

obj = null;

WeakSet - - -► Object

• Weak reference
• Object can be garbage collected
• Entry is automatically removed

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Why Is WeakSet Not Iterable?

The garbage collector can remove objects at any time.

If iteration were allowed, the collection could change while being traversed, leading to unpredictable behavior.

Therefore, WeakSet has:

• No for...of
• No forEach()
• No keys()
• No values()
• No entries()
• No size

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Comparison

Feature	Set	WeakSet
Stores primitives	Yes	No
Stores objects	Yes	Yes
Duplicate values	No	No
Iterable	Yes	No
size	Yes	No
clear()	Yes	No
Strong references	Yes	No
Automatic garbage collection	No	Yes

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Use Cases

Use Set when:

• Need unique values
• Need iteration
• Need collection size
• Need to store primitives

Use WeakSet when:

• Tracking objects only
• Avoiding memory leaks
• Tracking processed DOM elements
• Temporary object metadata

Example:

const processed = new WeakSet();

function process(element) {
    if (processed.has(element)) return;

    processed.add(element);

    // Process element...
}