Opal is a source to source compiler. It accepts ruby code as a string and generates javascript code which can be run in any environment. Generated code relies on the opal runtime which provides the class system and some other runtime helpers.
The compiler can be broken down into 3 separate stages:
The opal parser relies on the parser gem, see debug/development documentation there to know more about its internals: https://whitequark.github.io/parser/.
The opal compiler takes these sexps from the parser and generates ruby code from them. Each type of sexp has its own node type used to generate javascript. Each node creates an array of one or more fragments which are the concatenated together to form the final javascript. Fragments are used as they contain the generated code as well as a reference back to the original sexp which is useful for generating source maps afterwards.