If you are an Erlang developer you are likely familiar with the Riak storage engine, and how the MapReduce feature can evaluate arbitrary JavaScript expressions. Can you use this same code to do your own JavaScript evaluations for your Erlang projects? Yes, and it turns out to be surprisingly easily.

Getting your development environment running erlang_js is the first order of business. I’ve done all of my testing on a Mac OS X system, both Lion (10.7) and Mavericks (10.9). I use Erlang R14B04 for these tests, since that version is what we primarily target at the office.

If you want to manage multiple versions of Erlang on your OS X system, I highly recommend the erlbrew utility as a quick and easy way to maintain multiple versions. We’ve recently made it support R14 builds on OS X Mavericks, so there’s no excuse not to use it!

Assuming you have a working Erlang and OS X command-line development tools active, execute the following to checkout the erlang_js project and build it locally:

git clone git://github.com/basho/erlang_js.git
cd erlang_js
make all test

One of the most compelling uses of a JavaScript engine from another language like Erlang is the ability to pass arbitrarily deep objects as JSON so they can be evaluated. This allows your application to use JavaScript as a general-purpose expression language even if the objects are backed by an Erlang term. This is effectively what you get with Riak when using the MapReduce feature.

As an example, let’s imagine a deeply nested data structure that represents a server configuration. We’d like to execute a JavaScript expression against that data to find the first active network interface. Here’s an example of how you’d do it using erlang_js:

% Start the erlang_js application and driver. 
ok = application:start(erlang_js). 
{ok, JSDriver} = js_driver:new(). 

% Create a deeply-nested data structure to evaluate 
ServerObj = {struct, [{<<"id">>,<<"foobar.example.org">>}, {<<"type">>,<<"linux">>}, {<<"interfaces">>, [ {struct, [ {<<"name">>,<<"eth0">>}, {<<"ipaddress">>,<<"">>}, {<<"enabled">>,true}]}, {struct, [{<<"name">>,<<"eth1">>}, {<<"ipaddress">>,<<"">>}, {<<"enabled">>,true}]}, {struct, [{<<"name">>,<<"bridge0">>}, {<<"enabled">>,false}]} ]} ]}. 

% Evaluate our object with an arbitrary JavaScript expression 
JSFun = <<"function is_any_interface_enabled(Server) { for (var i = 0; i \< Server.interfaces.length; i++) { if (Server.interfaces[i].enabled) return true; } return false; }">>. 
js:call(JSDriver, JSFun, [ServerObj]).

The real trick is building your Erlang data structure properly so that the custom version of mochijson2 can encode the object as JSON. As you see from the examples, use tuples with a struct atom as the first member to specify an object. Arrays are just arrays.

I have noticed that it is very easy to confuse a single instance of the js_driver object and it is hosed until a new one is created, so consider that in how you structure your driver usage.