Adressing properties of an object

Question

This code only works, when I create the property 'area' after initialasing an object, not while creating it. How can I add a property that addresses the other properties of the same object WHILE creating an object, not after?

https://code.sololearn.com/W73eVOzTNYI2/?ref=app

Answer 1

https://code.sololearn.com/W7LINQibPSUz/#js