This is the Opal corelib implementation API documentation. The whole corelib is loaded upon require 'opal'.

The runtime.js documentation is available here (in master)

Note that 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.