How check an object's every key and value if null or undefined with ES6?

I'm trying to check an objects every entry's key and value in a if condition, but i cannot understand my result. What is the problem? Why is an undefined not false with "??" ?

const object = {undefined: "foo", B: "Bazz"}
console.log(Object.entries(object).every(([key, value]) => !!key && !!value))

//true

const object2 = {undefined: null, B: "Bazz"}
console.log(Object.entries(object2).every(([key, value]) => !!key && !!value)) 

//false

You cannot actually set the key to undefined. When you do { undefined: "foo" }, you are actually setting the key to the string "undefined". It is essentially the same as { "undefined": "foo" }

!!"undefined" && !!"foo" => true && true => true because non-empty strings are truthy

!!"undefined" && !!null => true && false => false because null is falsy