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WeakSet

In short

A WeakSet is a Set for objects that doesn't hold on to them tightly. Use it to mark objects as "seen" or "done" without causing memory leaks.

Like a Set, but can only hold objects (not primitives) and holds them weakly, allowing garbage collection once nothing else references them. Its weak references make it suitable for tracking object membership without keeping those objects alive solely because they are present in the collection.

The jaadu mascot as a teacher ticks a box on a clipboard with a giant pencil.

Think of it like this

Ticks on the attendance sheet

The teacher ticks names on today's sheet. If a student leaves college for good, their tick quietly disappears with them. You can ask "is this student ticked?", but you can't print the full list of ticks.

Watch it happen

Sending the welcome email only once

Your code

processed

has(user)

1/7First call. Is this exact user object in the WeakSet?

The big ideas

What you need to know

Objects only

Like WeakMap, a WeakSet can only hold objects. Adding a number or string throws a TypeError.

Just add, has, delete

No size, no looping. You can only ask about objects you already have in hand.

Doesn't keep things alive

If the only reference to an object is inside the WeakSet, the object can still be garbage collected.

Great for marking

Track which objects you've already processed, or which nodes you've visited while walking nested data.

See it in code

Walk through the code

1const processed = new WeakSet();2 3function sendWelcomeEmail(user) {4  if (processed.has(user)) return "Already sent";5  processed.add(user);6  return "Email sent to " + user.email;7}8 9const user = { email: "tanya@example.com" };10console.log(sendWelcomeEmail(user));  // "Email sent to tanya@example.com"11console.log(sendWelcomeEmail(user));  // "Already sent"12 13// When 'user' isn't referenced anywhere else, it can be collected.14// The WeakSet won't keep it alive.

1/4A WeakSet to remember which users are done.

You get a cheap "done" flag without adding an extra property to the user object, and without leaking memory.

Save yourself the pain

Common mistakes

Oops

Expecting weakSet.size or a for...of loop to work.

Do this instead

Neither exists on a WeakSet. If you need to count or list items, use a normal Set.

Oops

Checking with a new object that looks the same, like has({ id: 1 }).

Do this instead

WeakSet compares by identity. You must pass the exact same object you added.

Your mission

Stop double clicks

~15 min

Quick check

What does this log? const ws = new WeakSet(); ws.add({ id: 1 }); console.log(ws.has({ id: 1 }));

Go deeper

1Read a use case for WeakSet: tracking which objects have been 'processed' without leaking. Then apply the idea in a small example so the theory becomes practical.
2Compare WeakSet to Set — note WeakSet also isn't iterable. Write down when you would choose each option and what trade-off drives that choice.
3Understand this is a niche tool — most day-to-day code uses Set, not WeakSet.

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