> What version should I use? With backend stuff I am usually responsible for installing the versions of the code / language / libraries that I use. With browser

> What version should I use? With backend stuff I am usually responsible for installing the versions of the code / language / libraries that I use. With browsers what can I reasonably expect users to have in their browser? this is a tricky question. You need to test each feature you want to use on targeted browsers, really. Or use a specialized compiler that will compile your javascript to a former version of the language (a tool like babel does that). A site like https://caniuse.com https://caniuse.com can help you identify which feature works on what browser. But at the end of the day, it's like DOM API, your scripts need to be tested and errors monitored in production.