WeakSet.prototype.add()
Baseline
Widely available
This feature is well established and works across many devices and browser versions. It’s been available across browsers since September 2015.
The add() method of WeakSet instances inserts the specified value into this set, if it is not already present.
Try it
const weakset = new WeakSet();
const object = {};
weakset.add(object);
console.log(weakset.has(object));
// Expected output: true
try {
weakset.add(1);
} catch (error) {
console.log(error);
// Expected output (Chrome): TypeError: Invalid value used in weak set
// Expected output (Firefox): TypeError: WeakSet value must be an object, got 1
// Expected output (Safari): TypeError: Attempted to add a non-object key to a WeakSet
}
Syntax
js
add(value)
Parameters
value-
The value to add to the
WeakSetobject. Must be either an object or a non-registered symbol. Objects are compared by reference, not by value.
Return value
The WeakSet object.
Exceptions
TypeError-
Thrown if
valueis not an object or a non-registered symbol.
Examples
Using add()
js
const ws = new WeakSet();
ws.add(window); // add the window object to the WeakSet
ws.has(window); // true
// WeakSet only takes objects as arguments
ws.add(1);
// results in "TypeError: Invalid value used in weak set" in Chrome
// and "TypeError: 1 is not a non-null object" in Firefox
Specifications
| Specification |
|---|
| ECMAScript® 2026 Language Specification # sec-weakset.prototype.add |