is-weakref 
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Is this value a JS WeakRef? This module works cross-realm/iframe, and despite ES6 @@toStringTag.
Example
var isWeakRef = require('is-weakref');
assert(!isWeakRef(function () {}));
assert(!isWeakRef(null));
assert(!isWeakRef(function* () { yield 42; return Infinity; });
assert(!isWeakRef(Symbol('foo')));
assert(!isWeakRef(1n));
assert(!isWeakRef(Object(1n)));
assert(!isWeakRef(new Set()));
assert(!isWeakRef(new WeakSet()));
assert(!isWeakRef(new Map()));
assert(!isWeakRef(new WeakMap()));
assert(isWeakRef(new WeakRef({})));
class MyWeakRef extends WeakRef {}
assert(isWeakRef(new MyWeakRef({})));
Tests
Simply clone the repo, npm install, and run npm test