You can find more information regarding the core language API, here: http://api.haxe.org/js/
The following resources are worth checking out as a refresher:
The pixi.js extern can be downloaded via:
haxelib install pixijs
And if you visit the page on lib.haxe.org you can read more about the pixijs extern. For example: the maintainer is Adi Reddy Mora aka adireddy, how to install it (haxelib) where the original open source files are located (github), instructions and lots more.