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WeakSet

A Set variant that holds weak references to objects, allowing entries to be garbage-collected when no other references exist.

Since ES2024 Spec ↗

Syntax

const ws = new WeakSet([iterable]);

Parameters

NameTypeRequiredDescription
iterable Iterable<object> | null No An optional iterable of objects to pre-populate the set. Only objects and non-registered symbols are valid members.

Returns

WeakSet — A new WeakSet instance.

Throws

  • TypeError — When a primitive value is passed to add().

Examples

// Cycle detection in a linked list traversal
function hasCycle(head) {
  const seen = new WeakSet();
  let cur = head;
  while (cur) {
    if (seen.has(cur)) return true;
    seen.add(cur);
    cur = cur.next;
  }
  return false;
}
const a = { val: 1, next: null };
const b = { val: 2, next: null };
a.next = b; b.next = a; // cycle
console.log(hasCycle(a));
Output 
true

// Track processed DOM nodes without leaking memory
const processed = new WeakSet();

function process(node) {
  if (processed.has(node)) return;
  processed.add(node);
  // ... do work
  console.log('processed', node.id);
}
process({ id: 'btn-1' });
process({ id: 'btn-1' }); // different object reference — processes again
Output 
processed btn-1
processed btn-1

Notes

Like WeakMap, WeakSet is not enumerable — it has no `size`, `keys()`, `values()`, `entries()`, or `forEach()`. Its only methods are `add(obj)`, `has(obj)`, and `delete(obj)`. A WeakSet is appropriate when you need a "visited" or "seen" flag on objects without preventing those objects from being GC-collected when they go out of scope in the rest of the program.

See also