Is there any way to distinguish between an unset property and a property set to undefined? [duplicate]

Question

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 1

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

Answer 2

You can actually distinguish with extra condition before checking using in operator

if(a in testObject) {
  // it's there .. now check whether it is undefined or not
}

Answer 3

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