I think theres one https://developer.mozilla.org/en/JavaScript/Reference but for for things like what properties/functions a Textarea object will have, where can I get it?
I know textarea have properties like selectionStart, selectionEnd but what others? where can I find that info?
The Gecko DOM Reference is pretty complete.
For the dark side (IE-specific stuff), there's also Microsoft's HTML and DHTML Reference
And for cross-browser advice, consult QuirksMode (thanks Raynos)