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WeakMap.prototype.set()

Baseline Widely available

This feature is well established and works across many devices and browser versions. It’s been available across browsers since ⁨July 2015⁩.

The set() method of WeakMap instances adds a new entry with a specified key and value to this WeakMap, or updates an existing entry if the key already exists.

Try it

const weakmap = new WeakMap();
const object1 = {};
const object2 = {};

weakmap.set(object1, "foo");
weakmap.set(object2, "bar");

console.log(weakmap.get(object1));
// Expected output: "foo"

console.log(weakmap.get(object2));
// Expected output: "bar"

Syntax

set(key, value)

Parameters

key: The key of the entry to add to or modify within the WeakMap object. Must be either an object or a non-registered symbol. Object keys are compared by reference, not by value.
value: The value of the entry to add to or modify within the WeakMap object. Can be any value.

Return value

The WeakMap object.

Exceptions

TypeError: Thrown if key is not an object or a non-registered symbol.

Examples

Using set()

const wm = new WeakMap();
const obj = {};

// Add new elements to the WeakMap
wm.set(obj, "foo").set(window, "bar"); // chainable

// Update an element in the WeakMap
wm.set(obj, "baz");

// Using a non-registered symbol as key
const sym = Symbol("foo");
wm.set(sym, "baz");
wm.set(Symbol.iterator, "qux");

Specifications

Specification
ECMAScript® 2026 Language Specification> # sec-weakmap.prototype.set>

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
`set`	36	12	6 Before Firefox 38, this method threw a `TypeError` when the key parameter was not an object. This has been fixed in version 38 and later to return `false` as per the ES2015 standard.	23	8	36	6 Before Firefox for Android 38, this method threw a `TypeError` when the key parameter was not an object. This has been fixed in version 38 and later to return `false` as per the ES2015 standard.	24	8	3.0	37	8	1.0.0	1.0	0.12.0