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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

add(value)

Parameters

value: The value to add to the WeakSet object. 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 value is not an object or a non-registered symbol.

Examples

Using add()

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>

Browser compatibility

	Desktop					Mobile							Server
	Chrome	Edge	Firefox	Opera	Safari	Chrome Android	Firefox for Android	Opera Android	Safari on IOS	Samsung Internet	WebView Android	WebView on iOS	Bun	Deno	Node.js
`add`	36	12	34	23	9	36	34	24	9	3.0	37	9	1.0.0	1.0	0.12.0