This is quite obvious when you think about it, but it’s easy to miss nonetheless when you’re searching for something in a hurry and could lead to some problems:
It’s easy to tell which version you’re in, it’s right at the top:
With the server-side OM, a lot of times it didn’t matter which version you referenced because, unless you were looking for something about a new capability, the server-side OM was pretty well established and the core bits didn’t change very much.
This is not so with the JS OM. It is very much still evolving and even core bits are seeing massive changes. One case in point, the Web class had 14 methods in SharePoint 2010 but that count more than doubles in 2013 to 30 methods.
If you’re looking at the wrong page in MSDN, you’re going to miss a lot of interesting capabilities.