Parsing Ruby from JavaScript with opal-parser

Generally is best to precompile Ruby source files to JavaScript server-side but sometimes may become useful to be able to compile Ruby to JavaScript directly from JS.

Opal is able to compile its – pure Ruby – compiler to JavaScript (how cool is that!). The whole compiler chain is available in the stdlib as opal-parser.

require 'opal-parser'



opal-parser provides a partial implementation of Kernel#eval that only support the first parameter (String) and refuse any passed binding, lineno or filename.


require 'opal-parser'
eval "puts 'hello world!'"


Will fetch a remote URL (by means of a sync XMLHttpRequest) and evaluate its contents as Ruby code.


require 'opal-parser'
require_remote ''!

Opal.compile() and Opal.eval() (JavaScript)

After requiring opal-parser both Opal.compile() and Opal.eval() functions are added to the JavaScript API.

Opal.compile(string, options) (JavaScript) will forward the call to Opal.compile (Ruby) converting options from a plain JS object to a Ruby Hash. Opal.eval(string) will compile the given code to JavaScript and then pass it to the native eval() function.

Support for <script type="text/ruby">

When opal-parser is required it will search the page for any <script> tag with type text/ruby. If an src attribute is present will fetch and eval the file with Kernel#require_remote otherwise it will get the script tags contents and eval them with Kernel#eval.