Events

• You may have experience with events using jQuery

$("#albums").on("click", ".album",function(){
    //Do something with this album
});

Node Events

• Node events are similar in that you can listen for them using .on()
• Events you can listen for are emit()ed from objects
• If an object emits events it must be a child of EventEmitter

var EventEmitter = require('events').EventEmitter;

var attendant = new EventEmitter();

attendant.on('door bell', function(){
        console.log("Answer the door");
});

//at some random point

attendant.emit('door bell');

Node Events

• You can also pass relevant data with events

var EventEmitter = require('events').EventEmitter;

var attendant = new EventEmitter();

attendant.on('door bell', function(eventTime, eventRandNum){
        console.log("Answered the door for ", eventTime, " and random number", eventRandNum);
});

//at some random point

attendant.emit('door bell', Date.now(), Math.random());

Node Events

• Many of the node objects will emit events for your to listen to
  • http emits request events
  • The http request object will emit data as the request comes in

HTTP Server

• Setup a basic http server by listening for the request event

var http = require('http');
var server = http.createServer();
server.on('request', function(request, response){
        console.log("http request event fired!");
        response.end("http request event fired!");
});
server.listen(8000);

HTTP Server

• Alternatively
  • See the docs

var http = require('http');
http.createServer(function(request, response){
        console.log("http request event fired!");
        response.end("http request event fired!");
}).listen(8000);

Streams

• According to Node.js

A stream is an abstract interface implemented by various objects in Node. For example a request to an HTTP server is a stream, as is stdout. Streams are readable, writable, or both. All streams are instances of EventEmitter.

Streams

• A stream .write() will return whether or not there is more space in the buffer
  • If there is no space in the buffer you will begin exhausting RAM

Streams

• One option is to .pause() the stream you are reading from and waiting for the write stream to drain()

var http = require('http');

http.createServer(function(request, response){
        request.on('data', function(data){
                if(response.write(data) == false)
                    request.pause();
        });

        response.on('drain', function(){
            request.resume();
        })

        request.on('end', function(){
                response.writeHead(200);
                response.end();
        });
}).listen(8000);

Piping

• Another option is using a pipe
• Piping does handles the pause, resuming for you

var http = require('http');

http.createServer(function(request, response){
        request.pipe(response);
        request.on('end',function(){
            response.end();
        });
}).listen(8000);

File Uploads

• Pipe is useful for file uploads, why?

var fs = require('fs');
var http = require('http');

http.createServer(function(request, response) {
    var outFile = fs.createWriteStream("outFile");
    var uploadSize = request.headers['content-length'];
    var uploadedBytes = 0;

    request.pipe(outFile);

    request.on('data', function(data) {
        uploadedBytes += data.length;
        response.write(((uploadedBytes / uploadSize) * 100) + "%\n");
    });

    request.on('end', function() {
        response.end("Complete\n");
    });
}).listen(8000);

Line Counter

• Allows counting of large files quickly

let numLines = 0;

let lineReader = require("readline").createInterface({
    input: require('fs').createReadStream("amazon_reviews_us_Mobile_Apps_v1_00.tsv")
});

lineReader.on('line', function(line){
    numLines++;
});

lineReader.on('close', function(){
    console.log("We read the file and there are ", numLines, " in the file.");
});