WeakMap.prototype.has()

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

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The has() method of WeakMap instances returns a boolean indicating whether an entry with the specified key exists in this WeakMap or not.

Try it

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

weakmap.set(object1, "foo");

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

console.log(weakmap.has(object2));
// Expected output: false

Syntax

has(key)

Parameters

key: The key of the entry to test for presence in the WeakMap object. Object keys are compared by reference, not by value.

Return value

Returns true if an entry with the specified key exists in the WeakMap object; otherwise false. Always returns false if key is not an object or a non-registered symbol.

Examples

Using has()

const wm = new WeakMap();
wm.set(window, "foo");

wm.has(window); // returns true
wm.has("baz"); // returns false

Specifications

Specification
ECMAScript 2027 LanguageSpecification # sec-weakmap.prototype.has

Browser compatibility

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This page was last modified on Sep 22, 2025 by MDN contributors.

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Standard built-in objects
WeakMap
Constructor
1. WeakMap()
Instance methods
Inheritance
Object/Function
Static methods
Static properties
Instance methods
Instance properties
1. __proto__
2. constructor