cannot use "Set" in a session variable express

Question

I was trying to use Set as a session value, but for some reason I got this unexpected result. I'm making two requests from the client to this route.

app.post("/api/add_to_cart", (req, res) => {
    console.log(req.session.productList);
    if (req.session.productList === undefined)
        req.session.productList = new Set();
    console.log(req.session.productList);
    req.session.productList.add('one');
    console.log(req.session.productList);
    res.send();
});

output:

undefined
Set(0) {}
Set(1) { 'one' }

{}
{}
TypeError: req.session.productList.add is not a function

But if I use array insted of Set it works as expected

app.post("/api/add_to_cart", (req, res) => {
    console.log(req.session.productList);
    if (req.session.productList === undefined)
        req.session.productList = [];
    console.log(req.session.productList);
    req.session.productList.push('one');
    console.log(req.session.productList);
    res.send();
});

output:

undefined
[]
[ 'one' ]

[ 'one' ]
[ 'one' ]
[ 'one', 'one' ]

For some unknown reason the Set object is being changed to {}.

Answer 1

From the docs:

req.session

To store or access session data, simply use the request property req.session, which is (generally) serialized as JSON by the store, so nested objects are typically fine.
_{(highlighting mine)}

…and Sets/Maps do not support JSON (de)serialisation out of the box.

The builtin development-only MemoryStore does this as well.