node.js event emitter examples

EventEmitter is a property of the events module which comes with node.js installation. It allows you to emit an event by the using emit and listen on events by using the on.

A very simple example of using event emitter.

// initializing event emitter
var EventEmitter = require("events").EventEmitter;
var ee = new EventEmitter();


// Simplest example of event emitter
// listen for dummyEvent and console the data when dummyEvent is emitted.
ee.on("dummyEvent", function (data) {
	console.log(data);
});
console.log("This will be printed before the dummyEvent");
ee.emit("dummyEvent", "Hey this is a dummy event.");

A little more complex example of using event emitter.

// http for server and fs for file reading
var http = require("http");
var fs = require("fs");
// initializing event emitter
var EventEmitter = require("events").EventEmitter;
var ee = new EventEmitter();

// A litter more complex example of using event emitter
// create server and listen on port 3000
var server = http.createServer();
server.listen(3000);
console.log("Server on localhost:3000");

// the http has inherited the functions from EventEmitter
// listen on for the request event, when the request happens
// read a file and emit the error for errors and emit data if the read was successful.
// The error event will pass the response object and the err object
// The data event will pass the response object and the data object
server.on("request", function (req, res) {
	fs.readFile("./event_emitter.js", "utf-8", function (err, data) {
		if (err) {
			ee.emit("error", res, err);
		}
		else {
			ee.emit("data", res, data);
		}
	});
});

// listen on the error event, respond with the error when error happens
ee.on("error", function (res, err) {
	res.end(JSON.stringify(err));
});

// listen on the data event, respond with the data when the file was read successfully
ee.on("data", function (res, data) { 
	res.end(data); 
});