You probably don't need the answer as of now. You might not even see this comment. But to anyone who is interested in the explanation, here it is:

Storing an object as a key inside of an another object doesn't really work. Object keys in javascript are type of String (Even when you access a property of an object using a number i.e. obj[1] javacsript internally converts it to a String type). And since keys in an object should be of type string any object that is used as a key inside of another object will be turned into "[object Object]". That's why the check on line 4 fails to distinguish the other objects. The solution should work with maps though. Unlike in objects, you can use objects as keys in maps. For additional info on maps: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map

Because it's a "polyfill" of ES6's built-in BigInt class