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You can somehow use the firebase anonymous authentication if you do not want to really do any authentication. NB : You can not interact with the firebase realtime database and all other service without any form of authentication. Check here for more
I think it will be best if you start with ES5 and you can gradually transition to ES6 because ES6 is more of a syntactic sugar to ES5. If you want to get a head start you can check out this Hashnode original on how to move from zero to master javascript.