For Gecko there's Mozilla Developer Network
For IE there's MSDN.
For webkit there's...Apple Developer Connection?
There are a couple of JavaScript related documents on ADC, but nothing as comprehensive as MDN or MSDN. There's no reference. There's no way to look up methods of arrays or strings for webkit, or anything is there? So what, do we just assume it's the same as Gecko? IE?
How about the WebKit DOM Reference?
Update
Thanks for the comments - yes this now redirects.
Perhaps as others have said, one of the other browsers' references would be a good place to start - then for Safari's own DOM extensions/additions, there's the 'Safari DOM Additions Reference':
https://developer.apple.com/documentation/webkitjs
Otherwise it looks like it would be a case of finding your way through the confusing set of documents there in the Safari Developer Library: