But there are still issues of compatibility, and it’s by no means perfect but it’s improving. Your code can check for advanced browser features and should only use them if they’re supported, but let’s have a little digression about ECMAScript.
All modern versions of browsers support ECMAScript 5 features but what exactly defines a modern browser? A very handy checklist is here. It also shows you what features are supported in your browser in the first column. Greens are good.
If you think it’s time to update your learning or want a good grounding, also try the excellent free site http://www.codecademy.com and work through lots of the exercises. It uses gamification to incentivize you, and you earn badges whenever you’ve completed tutorials.