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 appends a new object to the end of this WeakSet.

Try it

const weakset1 = new WeakSet();
const object1 = {};

weakset1.add(object1);
console.log(weakset1.has(object1));
// Expected output: true

try {
  weakset1.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

Must be either an object or a non-registered symbol. The value to add to the WeakSet collection.

Return value

The WeakSet object.

Exceptions

TypeError

Thrown if value is 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® 2025 Language Specification
# sec-weakset.prototype.add

Browser compatibility

Report problems with this compatibility data on GitHub
desktopmobileserver
Chrome
Edge
Firefox
Opera
Safari
Chrome Android
Firefox for Android
Opera Android
Safari on iOS
Samsung Internet
WebView Android
WebView on iOS
Deno
Node.js
add

Legend

Tip: you can click/tap on a cell for more information.

Full support
Full support

See also