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Opal is a partial implementation of ruby designed to run in any javascript environment. The runtime is written in javascript, with all core libraries written directly in ruby with inline javascript to make them as fast as possible.

Wherever possible ruby objects are mapped directly onto their native javascript counterparts to speed up the generated code and to improve interopability with existing javascript libraries.

The opal compiler is a source-to-source compiler which outputs javascript which can then run on the core runtime.

Opal does not aim to be 100% comaptible with other ruby implementations, but does so where the generated code can be efficient on all modern web browsers - including older versions of IE and mobile devices.

Installing opal

Install the gem:

$ gem install opal

The opal command should then be available. To run the simple repl use:

opal irb


The quickest way to get opal running is to use the project generator. Simply run the command:

opal init my_project

replacing "my_project" with any name. This will make a "my_project" directory with a Rakefile, html document and libs needed for running opal in the browser.

Using opal in the browser

Opal runs directly in the browser, and is distributed as two files, opal.js and opal-parser.js. To just run precompiled code, just the opal.js runtime is required which includes the runtime and opals implementation of the ruby core library (pre compiled).

To evaluate ruby code directly in the browser, opal-parser.js is also required which will also load any ruby code found in script tags.


The Rakefile has a task to build your ruby project, so just run:

rake bundle

Open index.html in a browser, and now it should run. Edit, build and run to suit.

Project structure

This repo contains the code for the opal gem as well as the opal core library and runtime. Files inside bin/ and lib/ are the files that are used as part of the gem and run directly on your ruby environment.

corelib/ contains opal's core library implementation and is not used directly by the gem. These files are precompiled during development ready to be used in the gem or in a browser.

runtime/ contains opal's runtime written in javascript. It is not used directly by the gem, but is built ready to use in the js contexts that opal runs.

stdlib/ contains the stdlib files that opal comes packaged with. The gem does use these, but only as required. Opal does not include the full opal stdlib, and some parts are actually written in javascript for optimal performance. These can be require() at runtime.

opal.js and opal-parser.js are included in the gem, but not the source repo. They are the latest built versions of opal and its parser which are built before the gem is published.

Differences from ruby

Optional method_missing

To optimize method dispatch, method_missing is, by default, turned off in opal. It can easily be enabled by passing :method_missing => true in the parser options.

Immutable strings and removed symbols

All strings in opal are immutable to make them easier to map onto javascript strings. In opal a ruby string compiles directly into a js string without a wrapper so that they can be passed back between code bases easily. Also, symbol syntax is maintained, but all symbols just compile into javascript strings. The Symbol class is also therefore removed.

Unified Numeric class

All numbers in opal are members of the Numeric class. The Integer and Float classes are removed.

Unified Boolean class

Both true and false compile into their native javascript counterparts which means that they both become instances of the Boolean class and opal removes the TrueClass and FalseClass.

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