Lesson 84 – WeakSets in JavaScript

WeakSet` is a special kind of Set that only stores objects, and those objects are held weakly—meaning they don’t prevent garbage collection if there are no other references to them.

What is a WeakSet?

• Like a Set, a WeakSet lets you store unique values.
• But only objects (not primitives) can be added.
• Objects in a WeakSet are held weakly.
• WeakSet is not iterable and has no size property.

Creating a WeakSet

const ws = new WeakSet();

const obj1 = { name: "Alice" };
const obj2 = { name: "Bob" };

ws.add(obj1);
ws.add(obj2);

console.log(ws.has(obj1)); // true

ws.delete(obj2);
console.log(ws.has(obj2)); // false

Limitations of WeakSet

• You cannot list all elements.
• You can't use .forEach() or get .size.
• You can only use .add(), .has(), and .delete().

const ws = new WeakSet();
ws.add(1);  //  Error: Invalid value used in weak set

Use Case: Tracking Objects Without Preventing GC

WeakSets are good for storing metadata or tracking object states without interfering with memory cleanup.

Example:

const visitedNodes = new WeakSet();

function visit(node) {
  if (visitedNodes.has(node)) {
    console.log("Already visited");
    return;
  }

  visitedNodes.add(node);
  console.log("Visiting", node);
}

Try Your Hand

1. Create a WeakSet and add 2 different objects.
2. Check if a third object is in the set.
3. Delete one and check the result.
4. Try adding a primitive (and see the error).