Javascript set does not get returned in expressJs Json response

Question

I'm trying to return a simple set in the response.

const mySet1 = new Set();
mySet1.add(1);
console.log("mySet1", mySet1);
res.status(200).json({ my_set: mySet1 });

Even though, it gets logged correctly in the console:

mySet1 Set(1) { 1 }

In POSTMAN, this is what I see:

{
    "my_set": {}
}

Any idea what's going on?