>>Cat.prototype = new Mammal()

This is only useful if Mamaml has only methods and no properties which does not happen that often.

>> Cat.prototype.constructor = Cat

I never saw a practical usage of this one :-)

I think these days the standard way of doing inheritance should be -> MyClass.prototype = Object.create(baseClass.prototype)

After first glance it seems that React started way before modern frameworks emerged or am I wrong?

Just recently our company started developing WEB version of our product and our testers keep writing tests to verify that UI controls can correctly display unicode characters. Does it make much sense to do that if all our control if they are all HTML/JavaScript based?

You should ideally channel all character/encoding handling through one channel which can be tested and validated once. If there's a chance that every single page and widget may behave differently with regards to encodings, you have a bigger problem. You want to nail encodings once, then concentrate on other problems.

Having said that, more tests are hardly ever bad. Only if you start obsessing about and testing the same thing over and over I'd start to worry about some root causes.