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
| Specification | 
|---|
| ECMAScript Language Specification  # sec-weakset.prototype.has  | 
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