This is the Opal corelib implementation API documentation.
The whole corelib is loaded upon
runtime.js documentation is available here (in master)
require 'opal' will load all of the corelib, which is likely to
have a ton of stuff you don't need.
If you're concerned about runtime size, you can
require 'opal/base' and
require anything you need, or
require 'opal/mini' to have a working Ruby
without useless stuff.