nudge

Turn Node.js event emitters into Server Sent Event (aka EventSource) sources!

This module provides a function that wraps an event emitter, returning an express middleware that responds to browser EventSource requests. The wrapper can be configured to listen for particular events, optionally renaming and/or pre-processing them before forwarding to a client.

Usage

Basic

The most basic usage is to pass the function an event emitter, and an object with fields corresponding to events to listen on, and values set to true. For example:

var nudge = require('nudge');
var express = require('express');
var EventEmitter = require('events').EventEmitter;
 
var testEmitter = new EventEmitter();
 
// Listen for 'testEvent' and 'anotherTestEvent' events.
var testEmitterRelay = nudge(testEmitter, {
    testEvent: true,
    anotherTestEvent: true
});
 
var app = express();
 
// Navigate to localhost:3000.
app.get('/', function (req, res) {
    res.send(200);
});
 
// Host the events on a path. Use:
//     var emitter = new EventSource('/emitter');
//     emitter.addEventListener('testEvent', function (e) {
//         console.log(JSON.parse(e.data));
//     });
// To listen for 'testEvent' events and display destringified data.
 
app.get('/emitter', testEmitterRelay);
 
app.listen(3000);
 
// Intervals to emit sample events to send.
setInterval(function () {
    testEmitter.emit('testEvent', 'Hello, world!');
}, 1000);
 
setInterval(function () {
    testEmitter.emit('anotherTestEvent', 'Goodbye!');
}, 1500);

Advanced

You can rename and do some custom pre-processing on events before sending them through to the client. One important difference between EventSource and the Node.js EventEmitters is that EventSource listeners only have one data argument, whereas Node emitters may have many. Pre-processing gives you a chance to do something to the data from emitters like this in order to reduce them to a single argument. The callback of the preProcessor is used as the data argument. It's important to note that this is a success callback. Any errors should be handled by your function separately. By not calling the success callback, you can effectively filter results.

Take for example:

var testEmitterRelay = nudge(twitterEmitter, {
    testEvent: true,
    data: {
        name: 'tweet'
        preProcessor: function (args, successCallback) {
            successCallback({ user: args[0], tweet: args[1] });
        }
    }
});

Here the testEvent is just as before, but 'data' events are being renamed to 'tweet' and the two data arguments of emissions wrapped in a single object.

It's important to note that the callback passed to the pre-processor doesn't take an error. It's up to your pre-processor function to know what to do when errors occur.

Contributing

Contributions are welcome! Please see the contributing document.