WeakSet.prototype.has()
 The has() method returns a boolean indicating whether an object exists in a WeakSet or not. 
Syntax
has(value)
Parameters
- value
-  Required. The object to test for presence in the WeakSet.
Return value
- Boolean
-  Returns trueif an element with the specified value exists in theWeakSetobject; otherwisefalse.
Examples
Using the has method
var ws = new WeakSet(); var obj = {}; ws.add(window); mySet.has(window); // returns true mySet.has(obj); // returns false
Specifications
Browser compatibility
| Desktop | Mobile | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Chrome | Edge | Firefox | Internet Explorer | Opera | Safari | WebView Android | Chrome Android | Firefox for Android | Opera Android | Safari on IOS | Samsung Internet | |
| has | 36 | 12 | 34 | No | 23 | 9 | 37 | 36 | 34 | 24 | 9 | 3.0 | 
See also
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    https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/WeakSet/has