This is a JSON-RPC v2 handler middleware for the Express library on node.js. It is designed for use within routes.

First, simply include node-express-JSON-RPC2 in your configure/use statements before app.router:

var express = require('express'),
	jsonrpc = require('node-express-JSON-RPC2');

app.configure(function(){
	...
	app.use( express.jsonrpc() );
	...
	app.use( app.router );
});

Then wihin a route, use res.rpc() to handle a given method. The first argument is the method name and the second argument is the function that will be invoked to handle the request.

The first argument passed to the invokation function is the value of the "params" property from the RPC request. Unless the request is a notification, the second argument will be a function to handle the response. The response function will automatically wrap the data passed to it in the RPC response object containing the request ID and jsonrpc property.

app.post('/path/for/rpc/calls', function(req, res, next){
	// notification (no response expected)
	res.rpc('notification_method', function (params) {
		// do processing here
	});

	// non-notification (response expected)
	res.rpc('method_name', function(params, respond){
		// do processing here

		// if everything is OK return result object:
		respond({ result: resultData });

		// if something is wrong, return an error code:
		respond(jsonrpc.INVALID_PARAMS)
		// OR an extended error object:
		respond({
			error: {
				code: jsonrpc.INVALID_PARAMS,
				message: 'You gave me invalid parameters!',
				data: data
			}
		});
	});
});

The middleware provides standard error codes and messages defined by the JSON-RPC version 2 spec

You can reference error codes by using built-in variable names. For example, you may:

respond(jsonrpc.INVALID_REQUEST);

And your response will go out as:

{
	"jsonrpc": "2.0",
	"error": {
		"code": -32600,
		"message": "Invalid Request"
	},
	"id": null
}

The error code constants for reference:

Variable Name    Code     Message
------------------------------------------------
PARSE_ERROR      -32700  'Parse error'
INVALID_REQUEST  -32600  'Invalid request'
METHOD_NOT_FOUND -32601  'Method not found'
INVALID_PARAMS   -32602  'Invalid parameters'
INTERNAL_ERROR   -32603  'Internal error'

Note: For strict adherence to the specification, you cannot use bodyParser in conjuction with node-express-JSON-RPC2 because bodyParser will catch invalid JSON and return early, preventing a "Parse error" RPC response. If you are not terribly worried about strict compliance, using bodyParser is fine.