JavaScript WeakSet Data Structure

WeakSet vs Set

Here's a quick comparison:

Creating a WeakSet

A WeakSet is created using the new WeakSet() constructor. Unlike a regular Set, a WeakSet can only contain objects—not primitive values like numbers or strings.

1. Creating an Empty WeakSet

You can create an empty WeakSet and add object references later using the add() method.

const ws = new WeakSet();

const user = { name: 'Tom' };
ws.add(user);

console.log(ws.has(user));  // Output: true

const ws = new WeakSet();

const user = { name: 'Tom' };
ws.add(user);

console.log(ws.has(user));  // Output: true

2. Initializing WeakSet with Objects

You can also create a WeakSet from an array or other iterable containing objects.

const obj1 = {};
const obj2 = {};
const obj3 = {};

const ws = new WeakSet([obj1, obj2, obj3]);

console.log(ws.has(obj2));  // Output: true

const obj1 = {};
const obj2 = {};
const obj3 = {};

const ws = new WeakSet([obj1, obj2, obj3]);

console.log(ws.has(obj2));  // Output: true

3. Invalid Values Cause Errors

If you try to add a non-object value to a WeakSet, it will throw a TypeError.

const ws = new WeakSet();

ws.add(123);  // ❌ TypeError: Invalid value used in weak set

const ws = new WeakSet();

ws.add(123);  // ❌ TypeError: Invalid value used in weak set

Only object references are allowed. This ensures the "weak" referencing behavior works properly for garbage collection.

Accessing WeakSet Elements

Accessing elements in a WeakSet is different from a regular Set. Since WeakSet is not iterable, you can’t use loops, forEach(), or access elements by index.

The only way to check if an object exists in a WeakSet is by using the has() method.

1. Checking for an Object with has()

The has() method checks whether a specific object is present in the WeakSet.

const user1 = { name: 'Tom' };
const user2 = { name: 'Bob' };

const ws = new WeakSet([user1]);

console.log(ws.has(user1));  // Output: true
console.log(ws.has(user2));  // Output: false

const user1 = { name: 'Tom' };
const user2 = { name: 'Bob' };

const ws = new WeakSet([user1]);

console.log(ws.has(user1));  // Output: true
console.log(ws.has(user2));  // Output: false

2. Why WeakSets Aren’t Iterable

WeakSets are intentionally not iterable to support garbage collection. If JavaScript allowed iteration, it would have to prevent items from being collected, defeating the purpose of weak references.

As a result, methods like for...of, forEach(), or size don’t work on WeakSets:

const ws = new WeakSet([{ id: 1 }]);

// ❌ These will all throw errors:
for (const item of ws) {}         // TypeError
ws.forEach(() => {});             // TypeError
console.log(ws.size);             // undefined

const ws = new WeakSet([{ id: 1 }]);

// ❌ These will all throw errors:
for (const item of ws) {}         // TypeError
ws.forEach(() => {});             // TypeError
console.log(ws.size);             // undefined

Because of this limitation, WeakSet is primarily used in scenarios where you just need to track the presence of an object privately or temporarily.

Modifying WeakSet Elements

While WeakSet does not support direct access to its elements, you can modify its contents using three primary methods: add(), delete(), and indirectly through has().

1. Adding Elements with add()

The add() method inserts a new object into the WeakSet. If the object is already in the set, it will not be added again.

const ws = new WeakSet();

const item = { id: 1 };
ws.add(item);

console.log(ws.has(item));  // Output: true

const ws = new WeakSet();

const item = { id: 1 };
ws.add(item);

console.log(ws.has(item));  // Output: true

2. Deleting Elements with delete()

Use delete() to remove an object from the WeakSet. It returns true if the object was found and removed, or false if it wasn’t in the set.

const ws = new WeakSet();
const obj = { name: 'Test' };

ws.add(obj);
ws.delete(obj);  // true
console.log(ws.has(obj));  // false

const ws = new WeakSet();
const obj = { name: 'Test' };

ws.add(obj);
ws.delete(obj);  // true
console.log(ws.has(obj));  // false

3. Replacing an Object

Since WeakSets store only object references and don’t support indexing or iteration, replacing an object involves removing the old reference and adding a new one.

const ws = new WeakSet();
const oldObj = { key: 'old' };
const newObj = { key: 'new' };

ws.add(oldObj);

// Replace oldObj with newObj
if (ws.has(oldObj)) {
  ws.delete(oldObj);
  ws.add(newObj);
}

const ws = new WeakSet();
const oldObj = { key: 'old' };
const newObj = { key: 'new' };

ws.add(oldObj);

// Replace oldObj with newObj
if (ws.has(oldObj)) {
  ws.delete(oldObj);
  ws.add(newObj);
}

Remember, WeakSets do not update objects. You must manage object references manually if changes are needed.

WeakSet Methods

WeakSets have only three main methods:

• add(value): Adds an object to the WeakSet.
• has(value): Returns true if the object exists in the WeakSet.
• delete(value): Removes an object from the WeakSet.

const ws = new WeakSet();
const user = { name: 'Tom' };

ws.add(user);
console.log(ws.has(user));  // true

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

const ws = new WeakSet();
const user = { name: 'Tom' };

ws.add(user);
console.log(ws.has(user));  // true

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

Frequently Asked Questions

What's Next?

Next: Types of Arrays – explore the various ways arrays can be used and structured in JavaScript.