One can find it at https://developer.mozilla.
Although, it’s often best to use an internet search instead. Just use “MDN [term]” in the query, e.g. https://google.com/search?q=MDN+parseInt to search for parseInt function.
Also, we can use an internet search with phrases such as “RegExp MSDN” or “RegExp MSDN jscript”.
But being that formalized, it’s difficult to understand at first. So if you need the most trustworthy source of information about the language details, the specification is the right place. But it’s not for everyday use.
A new specification version is released every year. In-between these releases, the latest specification draft is at https://tc39.es/ecma262/.
To read about new bleeding-edge features, including those that are “almost standard” (so-called “stage 3”), see proposals at https://github.com/tc39/
Also, if you’re in developing for the browser, then there are other specs covered in the second part of the tutorial.