I haven’t looked at the source, but from the blog:
Calls to Erlang functions that the Elixir standard library make have to be reimplemented in JavaScript. There are many that are implemented, but there are more that need to be. This effects the availability of some Elixir standard library calls. A major contribution to ElixirScript would be to help with an implementation of an Erlang function that is blocking the use of an Elixir function you want to use!
