method preventExtensions javascript and defineProperty

Question

First of all Hello to everyone, and sorry for my English.
I would still take advantage of the expertise and availability of this community.
My question is realted to try to add properties to an object after preventExtensions method.
I know how this method works, this method makes an object not extensible, i can not add any property to this object
My question is: why if i try to add a property in this way (obj.prop), i'll find no errors in console and this property will be undefinded,
but if i try to add a new property with Object.defineproperty method i'll find an error in console?

<script>
       var obj={}
       Object.preventExtensions(obj)
       // i can not add other properies to this object
       console.log(Object.isExtensible(obj))//false

       // why if i try to add a property in this way i'll find undefined
       obj.b='bbb'
       console.log(obj.b)

       // and in this way i'll find an error
       Object.defineProperty(obj, 'w', { enumerable: true,configurable:true,writable:true,value:'www' });
</script>

Basically my question is about the different behavior between obj.prop and Object.defineProperty
Any suggestions will be read with pleasure, thank you all.

Answer 1

I imagine this behavior is browser specific. Even the documentation seems to be vague on the topic.

Attempting to add new properties to a non-extensible object will fail, either silently or by throwing a TypeError (most commonly, but not exclusively, when in strict mode).