Once you have a deep knowledge and experience applying core ideas in any language, framework, why bother learning another new fancy implementation flavor? If

Once you have a deep knowledge and experience applying core ideas in any language, framework, why bother learning another new fancy implementation flavor? If you would need to — you open docs, spend few days learning, playing with examples - and you ready to go. This is how I became JS developer one day with near zero prior JS experience. Okay, i'm not that smart and it took me few weeks instead of days ) But still, after java and ruby I didn't face any radically knew info or challenges getting on js train.