What does console.log(obj.hasOwnProperty('a'), obj.hasOwnProperty('toString')) print? JavaScript this, Prototypes and Classes

Answer, A. true false

A. true false

Correct. a is an own property of obj, while toString is only reachable through the inherited Object.prototype, so hasOwnProperty reports it as false.

B. true true

Assumes hasOwnProperty reports true for anything reachable on the object, whether it lives directly on the object or somewhere up its prototype chain. hasOwnProperty deliberately ignores inherited properties, it only checks the object itself.

C. false false

Assumes toString somehow fails to exist on obj at all, when it is reachable through Object.prototype and calling obj.toString() would work fine. hasOwnProperty is about where a property lives, not about whether it can be accessed.

D. false true

Assumes the order is reversed, that the built in toString counts as owned while the literal property a does not. a was written directly into the object literal, making it the clearly own property, while toString only comes from the shared prototype.

hasOwnProperty checks only the properties that live directly on the object it is called on, completely ignoring the prototype chain. a was set in the object literal, so it counts as own. toString is never defined on obj itself, it is inherited from Object.prototype, so hasOwnProperty correctly reports false even though obj.toString() would still work through inheritance.