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Is this value a JS WeakMap? This module works cross-realm/iframe, and despite ES6 @@toStringTag.

Example

```js var isWeakMap = require('is-weakmap'); assert(!isWeakMap(function () {})); assert(!isWeakMap(null)); assert(!isWeakMap(function* () { yield 42; return Infinity; }); assert(!isWeakMap(Symbol('foo'))); assert(!isWeakMap(1n)); assert(!isWeakMap(Object(1n)));

assert(!isWeakMap(new Set())); assert(!isWeakMap(new WeakSet())); assert(!isWeakMap(new Map()));

assert(isWeakMap(new WeakMap()));

class MyWeakMap extends WeakMap {} assert(isWeakMap(new MyWeakMap())); ```

Tests

Simply clone the repo, npm install, and run npm test