I'm using cookies to pass the session about the logged in user, and on the server I can set and get them fine:
res.cookie("foo", { bar: "baz" })
// later on
console.log(req.cookies.foo) // { bar: "baz" }
Now, I want to test it, and I need to set the cookie header, but I don't know how to convert an object, i.e. { bar: "baz" }, to the header string the cookie evaluates to

What's the algorithm used here?
(I'm using supertest but it shouldn't matter IMO)
In express by default the cookie is encoded, you need to pass the options to keep it as a string.
encode Function - A synchronous function used for cookie value encoding. Defaults to encodeURIComponent.
// Custom encoding
res.cookie("foo", { bar: "baz" }, {encode: String})
You can check here for more details.