api.js

exports.someMethod = function (arg1, arg2, callback) {
  // Do this for safety's sake, people might accidentally send us an extra arg
  arg1 = arguments[0];
  arg2 = arguments[1];
  callback = arguments[arguments.length - 1];

  callback(null, 'win!');
};

jsonrpc_call.js

// The call
{
  "method": "someMethod",
  "params": [ arg1, arg2 ],
  "id": new Date().getTime()
}

// The response
{
  "id": 12345, // The date we sent
  "result": "win!",
  "error": null
}