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A port of the Cap'n Proto runtime to JavaScript. Includes a schema compiler that generates JavaScript.


This is work in progress. It is currently based on an outdated version of Cap'n Proto, which means it doesn't include the recent object capability and RPC additions or any recent bugfixes. Also, dynamic access (at runtime, based on schemas) is not implemented and probably will never be, as JavaScript is itself a dynamic language.

It includes a mostly complete port of the Cap'n Proto test suite with over 100 test cases passing.

There are a number of open issues, including the following:

  • This isn't very optimized yet and might be terribly slow.

  • Currently you need to use Google Closure Compiler to link your code.

  • Functions are distributed over nested namespaces, which makes client code (pre-link) somewhat verbose.

  • Functions don't check their argument types and are mostly lacking Google Closure parameter annotations, which means it's pretty easy to shoot yourself in the foot.

  • Generated code is overly verbose and in places uses inconsistent naming.

  • Some of the code is a bit messy and redundant, in particular primitive getter/setters and capnp_list.js.

  • No support for Mozilla's int64 datatype or any of the common JavaScript bignum libraries. int64 are currently represented as an array of two int32s.

Getting Started

The build isn't very straighforward at the moment. You will need:

You need to edit the following files and adjust paths manually:

  • javascript/
  • javascript/tests/all_tests.html

Then try this:

$capnproto_js/c++/configure \
    --with-capnp-source=$capnproto/c++ \
    --with-capnp=$capnproto_build/capnp \
open test/all_tests.html # in your browser


All tests pass in current versions of Chrome (31), Firefox (25) and Safari (7). Other browsers have not been tested yet.