How do you determine if you can use certain code or not (crossbrowser, etc)

I was watching a course that was talking about spread operators, and how it is what the newer version of Javascript will use. I personally for example like using templating instead of the old string concatenation. I’m still not clear when I can use something or not, I mean in a profesional environment, client websites, etc. How do you guys check if your code is compatible with IE9 for example? Or if a certain functionality like templates can be used or not?

Thanks in advance.

http://caniuse.com/#search=templates

That’s great, thank you.

@Sergi-O Babel is a great tool to transpile the newer JavaScript syntax (ES6 and beyond) into ES5 which virtually all browsers understand. If you need to check if a specific functionality is available in a browser, CanIUse is a great tool and these extensions for Chrome and Atom make it easier to view support tables quickly in your browser or in Atom (if you use that) while coding.

Thank you as well, I knew about Babel, I was wondering about CSS rules too, and CanIUseIt covers that so I’m all set.