Extending the JavaScript built-in object is not a good idea because if browser/JS has decided that they will provide the same method that you have extended, then your method will be override and the JS implementation (which may be difference from yours) would take over.

So not extending in the first place is future proofing your codes, on the other side, If you decide to overwrite the browsers definition, any future developer working on your code won't be able to know above chnages. If you had to do the same then you have to make a library in your project and put these extending logic over there. So that you can easily marked those extensions and there won't be any conflicts.