javascript - Is there any way to distinguish between an unset property and a property set to undefined?

Say I have the object testObject = {a:
undefined}. If I then console.log(testObject.a), I
get undefined. But the same happens if I
console.log(testObject.b), which doesn't exist. Is there any
way in JavaScript to distinguish between a and
b here? I ask mostly out of curiosity, I have no use
case.

Answer

hasOwnProperty()
method returns a boolean indicating whether the object has the specified property as own
(not inherited) property.

In given case
-

 testObject = {a:
            undefined};

 testObject.hasOwnProperty('a')
            //true

 testObject.hasOwnProperty('b')
            //false