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The Node Beginner Book
A comprehensive Node.js tutorial
Manuel Kiessling
This book is for sale at http://leanpub.com/nodebeginner
This version was published on 2013-04-22

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©2011 - 2013 Manuel Kiessling

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Contents
About . . . . . . . . . . . . .
Status . . . . . . . . . . .
Intended audience . . . .
Structure of this document

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JavaScript and Node.js .
JavaScript and You . .
A word of warning . .
Server-side JavaScript .
“Hello World” . . . . .

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A full blown web application with Node.js . . . . . . .
The use cases . . . . . . . . . . . . . . . . . . . . . .
The application stack . . . . . . . . . . . . . . . . . .

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Building the application stack . . . . . . . . . . . . .
A basic HTTP server . . . . . . . . . . . . . . . . .
Analyzing our HTTP server . . . . . . . . . . . . .
Passing functions around . . . . . . . . . . . . . . .
How function passing makes our HTTP server work
Event-driven asynchronous callbacks . . . . . . . .
How our server handles requests . . . . . . . . . . .

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CONTENTS

Finding a place for our server module
What’s needed to “route” requests? .
Execution in the kingdom of verbs . .
Routing to real request handlers . . .
Making the request handlers respond
Serving something useful . . . . . . .
Handling POST requests . . . . . . .
Handling file uploads . . . . . . . . .

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Conclusion and outlook . . . . . . . . . . . . . . . . .

72

About
The aim of this document is to get you started with developing
applications for Node.js, teaching you everything you need to
know about “advanced” JavaScript along the way. It goes way
beyond your typical “Hello World” tutorial.

Status
You are reading the final version of this book, i.e., updates are
only done to correct errors or to reflect changes in new versions
of Node.js. It was last updated on April 22, 2013.
The code samples in this book are tested to work with Node.js
version 0.8.8.

Intended audience
This document will probably fit best for readers that have a
background similar to my own: experienced with at least one
object-oriented language like Ruby, Python, PHP or Java, only
little experience with JavaScript, and completely new to Node.js.
Aiming at developers that already have experience with other
programming languages means that this document won’t cover
really basic stuff like data types, variables, control structures and
the likes. You already need to know about these to understand
this document.
1

About

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However, because functions and objects in JavaScript are different from their counterparts in most other languages, these will
be explained in more detail.

Structure of this document
Upon finishing this document, you will have created a complete
web application which allows the users of this application to
view web pages and upload files.
Which, of course, is not exactly world-changing, but we will
go some extra miles and not only create the code that is “just
enough” to make these use cases possible, but create a simple,
yet complete framework to cleanly separate the different aspects
of our application. You will see what I mean in a minute.
We will start with looking at how JavaScript development in
Node.js is different from JavaScript development in a browser.
Next, we will stay with the good old tradition of writing a “Hello
World” application, which is a most basic Node.js application
that “does” something.
Then, we will discuss what kind of “real” application we want to
build, dissect the different parts which need to be implemented
to assemble this application, and start working on each of these
parts step-by-step.
As promised, along the way we will learn about some of the more
advanced concepts of JavaScript, how to make use of them, and
look at why it makes sense to use these concepts instead of those
we know from other programming languages.

JavaScript and Node.js
JavaScript and You
Before we talk about all the technical stuff, let’s take a moment
and talk about you and your relationship with JavaScript. This
chapter is here to allow you to estimate if reading this document
any further makes sense for you.
If you are like me, you started with HTML “development” long
ago, by writing HTML documents. You came along this funny
thing called JavaScript, but you only used it in a very basic way,
adding interactivity to your web pages every now and then.
What you really wanted was “the real thing”, you wanted to
know how to build complex web sites - you learned a programming language like PHP, Ruby, Java, and started writing
“backend” code.
Nevertheless, you kept an eye on JavaScript, you saw that with
the introduction of jQuery, Prototype and the likes, things got
more advanced in JavaScript land, and that this language really
was about more than window.open().
However, this was all still frontend stuff, and although it was nice
to have jQuery at your disposal whenever you felt like spicing up
a web page, at the end of the day you were, at best, a JavaScript
user, but not a JavaScript developer.
And then came Node.js. JavaScript on the server, how cool is
that?
3


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You decided that it’s about time to check out the old, new
JavaScript. But wait, writing Node.js applications is the one
thing; understanding why they need to be written the way they
are written means - understanding JavaScript. And this time for
real.
Here is the problem: Because JavaScript really lives two, maybe
even three lives (the funny little DHMTL helper from the mid90’s, the more serious frontend stuff like jQuery and the likes,
and now server-side), it’s not that easy to find information that
helps you to learn JavaScript the “right” way, in order to write
Node.js applications in a fashion that makes you feel you are not
just using JavaScript, you are actually developing it.
Because that’s the catch: you already are an experienced developer, you don’t want to learn a new technique by just hacking
around and mis-using it; you want to be sure that you are
approaching it from the right angle.
There is, of course, excellent documentation out there. But
documentation alone sometimes isn’t enough. What is needed
is guidance.
My goal is to provide a guide for you.

A word of warning
There are some really excellent JavaScript people out there. I’m
not one of them.
I’m really just the guy I talked about in the previous paragraph. I
know a thing or two about developing backend web applications,
but I’m still new to “real” JavaScript and still new to Node.js. I


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learned some of the more advanced aspects of JavaScript just
recently. I’m not experienced.
Which is why this is no “from novice to expert” book. It’s more
like “from novice to advanced novice”.
If I don’t fail, then this will be the kind of document I wish I had
when starting with Node.js.

Server-side JavaScript
The first incarnations of JavaScript lived in browsers. But this is
just the context. It defines what you can do with the language,
but it doesn’t say much about what the language itself can do.
JavaScript is a “complete” language: you can use it in many
contexts and achieve everything with it you can achieve with
any other “complete” language.
Node.js really is just another context: it allows you to run
JavaScript code in the backend, outside a browser.
In order to execute the JavaScript you intend to run in the
backend, it needs to be interpreted and, well, executed. This is
what Node.js does, by making use of Google’s V8 VM, the same
runtime environment for JavaScript that Google Chrome uses.
Plus, Node.js ships with a lot of useful modules, so you don’t have
to write everything from scratch, like for example something that
outputs a string on the console.
Thus, Node.js is really two things: a runtime environment and a
library.


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In order to make use of these, you need to install Node.js. Instead
of repeating the process here, I kindly ask you to visit the official
installation instructions¹. Please come back once you are up and
running.

“Hello World”
Ok, let’s just jump in the cold water and write our first Node.js
application: “Hello World”.
Open your favorite editor and create a file called helloworld.js.
We want it to write “Hello World” to STDOUT, and here is the
code needed to do that:
console.log("Hello World");

Save the file, and execute it through Node.js:
node helloworld.js

This should output Hello World on your terminal.
Ok, this stuff is boring, right? Let’s write some real stuff.
¹https://github.com/joyent/node/wiki/Installation

A full blown web
application with Node.js
The use cases
Let’s keep it simple, but realistic:
• The user should be able to use our application with a web
browser
• The user should see a welcome page when requesting
http://domain/start which displays a file upload form
• By choosing an image file to upload and submitting the
form, this image should then be uploaded to http://domain/upload,
where it is displayed once the upload is finished
Fair enough. Now, you could achieve this goal by googling and
hacking together something. But that’s not what we want to do
here.
Furthermore, we don’t want to write only the most basic code
to achieve the goal, however elegant and correct this code might
be. We will intentionally add more abstraction than necessary
in order to get a feeling for building more complex Node.js
applications.

7

A full blown web application with Node.js

8

The application stack
Let’s dissect our application. Which parts need to be implemented in order to fulfill the use cases?
• We want to serve web pages, therefore we need an HTTP
server
• Our server will need to answer differently to requests,
depending on which URL the request was asking for, thus
we need some kind of router in order to map requests to
request handlers
• To fullfill the requests that arrived at the server and have
been routed using the router, we need actual request
handlers
• The router probably should also treat any incoming POST
data and give it to the request handlers in a convenient
form, thus we need request data handling
• We not only want to handle requests for URLs, we also
want to display content when these URLs are requested,
which means we need some kind of view logic the request
handlers can use in order to send content to the user’s
browser
• Last but not least, the user will be able to upload images,
so we are going to need some kind of upload handling
which takes care of the details
Let’s think a moment about how we would build this stack with
PHP. It’s not exactly a secret that the typical setup would be an
Apache HTTP server with mod_php5 installed.
Which in turn means that the whole “we need to be able to serve

A full blown web application with Node.js

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web pages and receive HTTP requests” stuff doesn’t happen
within PHP itself.
Well, with node, things are a bit different. Because with Node.js,
we not only implement our application, we also implement the
whole HTTP server. In fact, our web application and its web
server are basically the same.
This might sound like a lot of work, but we will see in a moment
that with Node.js, it’s not.
Let’s just start at the beginning and implement the first part of
our stack, the HTTP server.

Building the application
stack
A basic HTTP server
When I arrived at the point where I wanted to start with my first
“real” Node.js application, I wondered not only how to actually
code it, but also how to organize my code.
Do I need to have everything in one file? Most tutorials on the
web that teach you how to write a basic HTTP server in Node.js
have all the logic in one place. What if I want to make sure that
my code stays readable the more stuff I implement?
Turns out, it’s relatively easy to keep the different concerns of
your code separated, by putting them in modules.
This allows you to have a clean main file, which you execute
with Node.js, and clean modules that can be used by the main
file and among each other.
So, let’s create a main file which we use to start our application,
and a module file where our HTTP server code lives.
My impression is that it’s more or less a standard to name your
main file index.js. It makes sense to put our server module into
a file named server.js.
Let’s start with the server module. Create the file server.js in the
root directory of your project, and fill it with the following code:

10

Building the application stack

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var http = require("http");
http.createServer(function(request, response) {
response.writeHead(200, {"Content-Type": "text/plain"});
response.write("Hello World");
response.end();
}).listen(8888);

That’s it! You just wrote a working HTTP server. Let’s prove it
by running and testing it. First, execute your script with Node.js:
node server.js

Now, open your browser and point it at http://localhost:8888/².
This should display a web page that says “Hello World”.
That’s quite interesting, isn’t it. How about talking about what’s
going on here and leaving the question of how to organize our
project for later? I promise we’ll get back to it.

Analyzing our HTTP server
Well, then, let’s analyze what’s actually going on here.
The first line requires the http module that ships with Node.js
and makes it accessible through the variable http.
We then call one of the functions the http module offers: createServer. This function returns an object, and this object has a
²http://localhost:8888/


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method named listen, and takes a numeric value which indicates
the port number our HTTP server is going to listen on.
Please ignore for a second the function definition that follows
the opening bracket of http.createServer.
We could have written the code that starts our server and makes
it listen at port 8888 like this:
var server = http.createServer();
server.listen(8888);

That would start an HTTP server listening at port 8888 and doing
nothing else (not even answering any incoming requests).
The really interesting (and, if your background is a more conservative language like PHP, odd looking) part is the function
definition right there where you would expect the first parameter
of the createServer() call.
Turns out, this function definition IS the first (and only) parameter we are giving to the createServer() call. Because in JavaScript,
functions can be passed around like any other value.

Passing functions around
You can, for example, do something like this:


13

function say(word) {
console.log(word);
}
function execute(someFunction, value) {
someFunction(value);
}
execute(say, "Hello");

Read this carefully! What we are doing here is, we pass the
function say as the first parameter to the execute function. Not
the return value of say, but say itself!
Thus, say becomes the local variable someFunction within execute, and execute can call the function in this variable by issuing
someFunction() (adding brackets).
Of course, because say takes one parameter, execute can pass
such a parameter when calling someFunction.
We can, as we just did, pass a function as a parameter to another
function by its name. But we don’t have to take this indirection of
first defining, then passing it - we can define and pass a function
as a parameter to another function in-place:
function execute(someFunction, value) {
someFunction(value);
}
execute(function(word){ console.log(word) }, "Hello");


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We define the function we want to pass to execute right there at
the place where execute expects its first parameter.
This way, we don’t even need to give the function a name, which
is why this is called an anonymous function.
This is a first glimpse at what I like to call “advanced” JavaScript,
but let’s take it step by step. For now, let’s just accept that in
JavaScript, we can pass a function as a parameter when calling
another function. We can do this by assigning our function to a
variable, which we then pass, or by defining the function to pass
in-place.

How function passing makes our HTTP
server work
With this knowledge, let’s get back to our minimalistic HTTP
server:
http.createServer(function(request, response) {
response.end();
}).listen(8888);

By now it should be clear what we are actually doing here: we
pass the createServer function an anonymous function.
We could achieve the same by refactoring our code to:


15

function onRequest(request, response) {
response.end();
}
http.createServer(onRequest).listen(8888);

Maybe now is a good moment to ask: Why are we doing it that
way?

Event-driven asynchronous callbacks
To understand why Node.js applications have to be written this
way, we need to understand how Node.js executes our code.
Node’s approach isn’t unique, but the underlying execution
model is different from runtime environments like Python, Ruby,
PHP or Java.
Let’s take a very simple piece of code like this:
var result = database.query("SELECT * FROM hugetable");

Please ignore for now that we haven’t actually talked about
connecting to databases before - it’s just an example. The first
line queries a database for lots of rows, the second line puts
“Hello World” to the console.


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Let’s assume that the database query is really slow, that it has to
read an awful lot of rows, which takes several seconds.
The way we have written this code, the JavaScript interpreter of
Node.js first has to read the complete result set from the database,
and then it can execute the console.log() function.
If this piece of code actually was, say, PHP, it would work the
same way: read all the results at once, then execute the next line
of code. If this code would be part of a web page script, the user
would have to wait several seconds for the page to load.
However, in the execution model of PHP, this would not become
a “global” problem: the web server starts its own PHP process for
every HTTP request it receives. If one of these requests results in
the execution of a slow piece of code, it results in a slow page load
for this particular user, but other users requesting other pages
would not be affected.
The execution model of Node.js is different - there is only one
single process. If there is a slow database query somewhere in
this process, this affects the whole process - everything comes to
a halt until the slow query has finished.
To avoid this, JavaScript, and therefore Node.js, introduces the
concept of event-driven, asynchronous callbacks, by utilizing an
event loop.
We can understand this concept by analyzing a rewritten version
of our problematic code:


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database.query("SELECT * FROM hugetable", function(rows) {
var result = rows;
});

Here, instead of expecting database.query() to directly return
a result to us, we pass it a second parameter, an anonymous
function.
In its previous form, our code was synchronous: first do the
database query, and only when this is done, then write to the
console.
Now, Node.js can handle the database request asynchronously.
Provided that database.query() is part of an asynchronous library, this is what Node.js does: just as before, it takes the query
and sends it to the database. But instead of waiting for it to be
finished, it makes a mental note that says “When at some point
in the future the database server is done and sends the result of
the query, then I have to execute the anonymous function that
was passed to database.query().”
Then, it immediately executes console.log(), and afterwards, it
enters the event loop. Node.js continuously cycles through this
loop again and again whenever there is nothing else to do,
waiting for events. Events like, e.g., a slow database query finally
delivering its results.
This also explains why our HTTP server needs a function it can
call upon incoming requests - if Node.js would start the server
and then just pause, waiting for the next request, continuing only
when it arrives, that would be highly inefficent. If a second user
requests the server while it is still serving the first request, that


18

second request could only be answered after the first one is done
- as soon as you have more than a handful of HTTP requests per
second, this wouldn’t work at all.
It’s important to note that this asynchronous, single-threaded,
event-driven execution model isn’t an infinitely scalable performance unicorn with silver bullets attached. It is just one of
several models, and it has its limitations, one being that as of
now, Node.js is just one single process, and it can run on only one
single CPU core. Personally, I find this model quite approachable, because it allows to write applications that have to deal
with concurrency in an efficient and relatively straightforward
manner.
You might want to take the time to read Felix Geisendoerfer’s
excellent post Understanding node.js³ for additional background
explanation.
Let’s play around a bit with this new concept. Can we prove
that our code continues after creating the server, even if no
HTTP request happened and the callback function we passed
isn’t called? Let’s try it:

³http://debuggable.com/posts/understanding-node-js:4bd98440-45e4-4a9a-8ef70f7ecbdd56cb


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console.log("Request received.");
response.end();
}
console.log("Server has started.");

Note that I use console.log to output a text whenever the onRequest function (our callback) is triggered, and another text right
after starting the HTTP server.
When we start this (node server.js, as always), it will immediately
output “Server has started.” on the command line. Whenever
we request our server (by opening http://localhost:8888/⁴ in our
browser), the message “Request received.” is printed on the
command line.
Event-driven asynchronous server-side JavaScript with callbacks
in action :-)
(Note that our server will probably write “Request received.”
to STDOUT two times upon opening the page in a browser.
That’s because most browser will try to load the favicon by
requesting http://localhost:8888/favicon.ico whenever you open
http://localhost:8888/).
⁴http://localhost:8888/


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How our server handles requests
Ok, let’s quickly analyze the rest of our server code, that is, the
body of our callback function onRequest().
When the callback fires and our onRequest() function gets triggered, two parameters are passed into it: request and response.
Those are objects, and you can use their methods to handle the
details of the HTTP request that occured and to respond to the
request (i.e., to actually send something over the wire back to the
browser that requested your server).
And our code does just that: Whenever a request is received, it
uses the response.writeHead() function to send an HTTP status
200 and content-type in the HTTP response header, and the
response.write() function to send the text “Hello World” in the
HTTP response body.
At last, we call response.end() to actually finish our response.
At this point, we don’t care for the details of the request, which
is why we don’t use the request object at all.

Finding a place for our server module
Ok, I promised we will get back to how to organize our application. We have the code for a very basic HTTP server in
the file server.js, and I mentioned that it’s common to have a
main file called index.js which is used to bootstrap and start our
application by making use of the other modules of the application
(like the HTTP server module that lives in server.js).


21

Let’s talk about how to make server.js a real Node.js module that
can be used by our yet-to-be-written index.js main file.
As you may have noticed, we already used modules in our code,
like this:
...
http.createServer(...);

Somewhere within Node.js lives a module called “http”, and we
can make use of it in our own code by requiring it and assigning
the result of the require to a local variable.
This makes our local variable an object that carries all the public
methods the http module provides.
It’s common practice to choose the name of the module for the
name of the local variable, but we are free to choose whatever
we like:
var foo = require("http");
...
foo.createServer(...);

Fine, it’s clear how to make use of internal Node.js modules. How
do we create our own modules, and how do we use them?


22

Let’s find out by turning our server.js script into a real module.
Turns out, we don’t have to change that much. Making some
code a module means we need to export those parts of its
functionality that we want to provide to scripts that require our
module.
For now, the functionality our HTTP server needs to export is
simple: scripts requiring our server module simply need to start
the server.
To make this possible, we will put our server code into a function
named start, and we will export this function:
function start() {
console.log("Request received.");
response.end();
}
}
exports.start = start;

This way, we can now create our main file index.js, and start
our HTTP there, although the code for the server is still in our
server.js file.


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Create a file index.js with the following content:
var server = require("./server");
server.start();

As you can see, we can use our server module just like any
internal module: by requiring its file and assigning it to a
variable, its exported functions become available to us.
That’s it. We can now start our app via our main script, and it
still does exactly the same:
node index.js

Great, we now can put the different parts of our application into
different files and wire them together by making them modules.
We still have only the very first part of our application in place:
we can receive HTTP requests. But we need to do something with
them - depending on which URL the browser requested from our
server, we need to react differently.
For a very simple application, you could do this directly within
the callback function onRequest(). But as I said, let’s add a bit
more abstraction in order to make our example application a bit
more interesting.
Making different HTTP requests point at different parts of our
code is called “routing” - well, then, let’s create a module called
router.


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What’s needed to “route” requests?
We need to be able to feed the requested URL and possible
additional GET and POST parameters into our router, and based
on these the router then needs to be able to decide which code to
execute (this “code to execute” is the third part of our application:
a collection of request handlers that do the actual work when a
request is received).
So, we need to look into the HTTP requests and extract the
requested URL as well as the GET/POST parameters from them.
It could be argued if that should be part of the router or part of
the server (or even a module of its own), but let’s just agree on
making it part of our HTTP server for now.
All the information we need is available through the request
object which is passed as the first parameter to our callback
function onRequest(). But to interpret this information, we need
some additional Node.js modules, namely url and querystring.
The url module provides methods which allow us to extract the
different parts of a URL (like e.g. the requested path and query
string), and querystring can in turn be used to parse the query
string for request parameters:


25

url.parse(string).query
|
url.parse(string).pathname
|
|
|
|
|
------ ------------------http://localhost:8888/start?foo=bar&hello=world
------|
|
|
|
querystring(string)["foo"]
|
|
querystring(string)["hello"]

We can, of course, also use querystring to parse the body of a
POST request for parameters, as we will see later.
Let’s now add to our onRequest() function the logic needed to
find out which URL path the browser requested:
var url = require("url");
function start() {
var pathname = url.parse(request.url).pathname;
console.log("Request for " + pathname + " received.");
response.end();
}


26

}

Fine. Our application can now distinguish requests based on
the URL path requested - this allows us to map requests to our
request handlers based on the URL path using our (yet to be
written) router.
In the context of our application, it simply means that we will
be able to have requests for the /start and /upload URLs handled
by different parts of our code. We will see how everything fits
together soon.
Ok, it’s time to actually write our router. Create a new file called
router.js, with the following content:
function route(pathname) {
console.log("About to route a request for " + pathname);
}
exports.route = route;

Of course, this code basically does nothing, but that’s ok for now.
Let’s first see how to wire together this router with our server
before putting more logic into the router.
Our HTTP server needs to know about and make use of our
router. We could hard-wire this dependency into the server,


27

but because we learned the hard way from our experience with
other programming languages, we are going to loosely couple
server and router by injecting this dependency (you may want
to read Martin Fowlers excellent post on Dependency Injection⁵
for background information).
Let’s first extend our server’s start() function in order to enable
us to pass the route function to be used by parameter:
function start(route) {
route(pathname);
response.end();
}
}
⁵http://martinfowler.com/articles/injection.html


28

And let’s extend our index.js accordingly, that is, injecting the
route function of our router into the server:
var router = require("./router");
server.start(router.route);

Again, we are passing a function, which by now isn’t any news
for us.
If we start our application now (node index.js, as always), and
request an URL, you can now see from the application’s output
that our HTTP server makes use of our router and passes it the
requested pathname:
bash$ node index.js
Request for /foo received.
About to route a request for /foo

(I omitted the rather annoying output for the /favicon.ico request).

Execution in the kingdom of verbs
May I once again stray away for a while and talk about functional programming again?
Passing functions is not only a technical consideration. With
regard to software design, it’s almost philosophical. Just think


29

about it: in our index file, we could have passed the router object
into the server, and the server could have called this object’s
route function.
This way, we would have passed a thing, and the server would
have used this thing to do something. Hey, router thing, could
you please route this for me?
But the server doesn’t need the thing. It only needs to get
something done, and to get something done, you don’t need
things at all, you need actions. You don’t need nouns, you need
verbs.
Understanding the fundamental mind-shift that’s at the core
of this idea is what made me really understand functional
programming.
And I did understand it when reading Steve Yegge’s masterpiece
Execution in the Kingdom of Nouns⁶. Go read it now, really. It’s
one of the best writings related to software I ever had the pleasure
to encounter.

Routing to real request handlers
Back to business. Our HTTP server and our request router are
now best friends and talk to each other as we intended.
Of course, that’s not enough. “Routing” means, we want to
handle requests to different URLs differently. We would like to
have the “business logic” for requests to /start handled in another
function than requests to /upload.
⁶http://steve-yegge.blogspot.com/2006/03/execution-in-kingdom-of-nouns.html


30

Right now, the routing “ends” in the router, and the router is not
the place to actually “do” something with the requests, because
that wouldn’t scale well once our application becomes more
complex.
Let’s call these functions, where requests are routed to, request
handlers. And let’s tackle those next, because unless we have
these in place there isn’t much sense in doing anything with the
router for now.
New application part, new module - no surprise here. Let’s create
a module called requestHandlers, add a placeholder function
for every request handler, and export these as methods of the
module:
function start() {
console.log("Request handler 'start' was called.");
}
function upload() {
console.log("Request handler 'upload' was called.");
}
exports.upload = upload;

This allows us to wire the request handlers into the router, giving
our router something to route to.
At this point we need to make a decision: do we hard-code
usage of the requestHandlers module into the router, or do we
want a bit more dependency injection? Although dependency


31

injection, like every other pattern, shouldn’t be used only for the
sake of using it, in this case it makes sense to loosely couple the
router and its request handlers, and thus making the router really
reusable.
This means we need to pass the request handlers from our server
into our router, but this feels even more wrong, which is why we
should go the whole way and pass them to the server from our
main file, and passing it on to the router from there.
How are we going to pass them? Right now we have two
handlers, but in a real application, this number is going to
increase and vary, and we sure don’t want to fiddle around
mapping requests to handlers in the router anytime a new URL /
request handler is added. And having some if request == x then
call handler y in the router would be more than ugly.
A varying number of items, each mapped to a string (the requested URL)? Well, sounds like an associative array would be a
perfect fit.
Well, this finding is slightly disappointed by the fact that JavaScript
doesn’t provide associative array - or does it? Turns out, it’s
actually objects that we want to use if we need an associative
array!
There’s a nice introduction to this at http://msdn.microsoft.com/enus/magazine/cc163419.aspx⁷, let me quote the relevant part:
In C++ or C#, when weâ€™re talking about objects,
we’re referring to instances of classes or structs.
⁷http://msdn.microsoft.com/en-us/magazine/cc163419.aspx


32

Objects have different properties and methods, depending on which templates (that is, classes) they
are instantiated from. That’s not the case with JavaScript
objects. In JavaScript, objects are just collections of
name/value pairs - think of a JavaScript object as a
dictionary with string keys.
If JavaScript objects are just collections of name/value pairs, how
can they have methods? Well, the values can be strings, numbers
etc. - or functions!
Ok, now finally back to the code. We decided we want to pass
the list of requestHandlers as an object, and in order to achieve
loose coupling we want to inject this object into the route().
Let’s start with putting the object together in our main file
index.js:
var requestHandlers = require("./requestHandlers");
var handle = {}
handle["/"] = requestHandlers.start;
handle["/start"] = requestHandlers.start;
handle["/upload"] = requestHandlers.upload;
server.start(router.route, handle);

Although handle is more of a “thing” (a collection of request
handlers), I propose we name it like a verb, because this will
result in a fluent expression in our router, as we will see soon.


33

As you can see, it’s really simple to map different URLs to the
same request handler: by adding a key/value pair of ”/” and
requestHandlers.start, we can express in a nice and clean way
that not only requests to /start, but also requests to / shall be
handled by the start handler.
After defining our object, we pass it into the server as an
additional parameter. Let’s change our server.js to make use of
it:
function start(route, handle) {
route(handle, pathname);
response.end();
}
}

We’ve added the handle parameter to our start() function, and


34

pass the handle object on to the route() callback, as its first
parameter.
Let’s change the route() function accordingly, in our router.js file:
function route(handle, pathname) {
if (typeof handle[pathname] === 'function') {
handle[pathname]();
} else {
console.log("No request handler found for " + pathname);
}
}

What we do here is, we check if a request handler for the given
pathname exists, and if it does, we simply call the according
function. Because we can access our request handler functions
from our object just as we would access an element of an
associative array, we have this nice fluent handle[pathname]();
expression I talked about earlier: “Please, handle this pathname”.
Ok, that’s all we need to wire server, router, and request handlers together! When starting our application and requesting
http://localhost:8888/start⁸ in our browser, we can prove that the
correct request handler was indeed called:

⁸http://localhost:8888/start


35

Server has started.
Request for /start received.
About to route a request for /start
Request handler 'start' was called.

And opening http://localhost:8888/⁹ in our browser proves that
these requests, too, are indeed handled by the start request
handler:
Request for / received.
About to route a request for /
Request handler 'start' was called.

Making the request handlers respond
Beautiful. Now if only the request handlers could actually send
something back to the browser, that would be even better, right?
Remember, the “Hello World” your browser displays upon requesting a page still comes from the onRequest function in our
server.js file.
“Handling request” means “answering requests” after all, thus we
need to enable our request handlers to speak with the browser
just like our onRequest function does.

How to not do it
The straight-forward approach we - as developers with a background in PHP or Ruby - might want to follow is actually very
⁹http://localhost:8888/


36

deceitful: it works like a charm, seems to make a lot of sense, and
then suddenly screws things up when we don’t expect it.
What I mean by “straight-forward approach” is this: make the
request handlers return() the content they want to display to the
user, and send this response data in the onRequest function back
to the user.
Let’s just do this, and then see why it’s not such an overly good
idea.
We start with the request handlers and make them return what
we would like to display in the browser. We need to change
requestHandlers.js to this:
function start() {
return "Hello Start";
}
function upload() {
return "Hello Upload";
}

Good. Likewise, the router needs to return to the server what
the request handlers return to him. We therefore need to edit
router.js like this:


37

function route(handle, pathname) {
return handle[pathname]();
} else {
return "404 Not found";
}
}

As you can see, we also return some text if the request could not
be routed.
And last but not least, we need to refactor our server to make
it respond to the browser with the content the request handlers
returned via the router, transforming server.js into:
var content = route(handle, pathname)
response.write(content);
response.end();


38

}
}

If we start our rewritten application, everything works like a
charm: requesting http://localhost:8888/start¹⁰ results in “Hello
Start” being displayed in the browser, requesting http://localhost:8888/upload¹¹
gives us “Hello Upload”, and http://localhost:8888/foo¹² produces
“404 Not found”.
Ok, then why is that a problem? The short answer: because we
will run into problems if one the request handlers wants to make
use of a non-blocking operation in the future.
Let’s take a bit more time for the long answer.

Blocking and non-blocking
As said, the problems will arise when we include non-blocking
operations in the request handlers. But let’s talk about blocking
operations first, then about non-blocking operations.
And instead of trying to explain what “blocking” and “nonblocking” means, let’s demonstrate ourselves what happens if we
add a blocking operation to our request handlers.
¹⁰http://localhost:8888/start
¹¹http://localhost:8888/upload
¹²http://localhost:8888/foo


39

To do this, we will modify our start request handler to make it
wait 10 seconds before returning its “Hello Start” string. Because
there is no such thing as sleep() in JavaScript, we will use a clever
hack for that.
Please modify requestHandlers.js as follows:
function start() {
function sleep(milliSeconds) {
var startTime = new Date().getTime();
while (new Date().getTime() < startTime + milliSeconds);
}
sleep(10000);
return "Hello Start";
}
function upload() {
}

Just to make clear what that does: when the function start() is
called, Node.js waits 10 seconds and only then returns “Hello
Start”. When calling upload(), it returns immediately, just like
before.


40

(Of course, you should imagine that instead of sleeping for 10
seconds, there would be a real life blocking operation in start(),
like some sort of long-running computation.)
Let’s see what this change does.
As always, we need to restart our server. This time, I ask you
to follow a slightly complex “protocol” in order to see what
happens: First, open two browser windows or tabs. In the first
browser window, please enter http://localhost:8888/start¹³ into
the address bar, but do not yet open this url!
In the second browser window’s address bar, enter http://localhost:8888/upload¹⁴,
and again, please do not yet hit enter.
Now, do as follows: hit enter on the first window (“/start”), then
quickly change to the second window (“/upload”) and hit enter,
too.
What you will notice is this: The /start URL takes 10 seconds
to load, as we would expect. But the /upload URL also takes
10 seconds to load, although there is no sleep() in the according
request handler.
Why? Because start() contains a blocking operation. We already
talked about Node’s execution model - expensive operations are
ok, but we must take care to not block the Node.js process with
them. Instead, whenever expensive operations must be executed,
these must be put in the background, and their events must be
handled by the event loop.
And we will now see why the way we constructed the “request
handler response handling” in our application doesn’t allow us
¹³http://localhost:8888/start
¹⁴http://localhost:8888/upload


41

to make proper use of non-blocking operations.
Once again, let’s try to experience the problem first-hand by
modifying our application.
We are going to use our start request handler for this again.
Please modify it to reflect the following (file requestHandlers.js):
var exec = require("child_process").exec;
function start() {
var content = "empty";
exec("ls -lah", function (error, stdout, stderr) {
content = stdout;
});
return content;
}
function upload() {
}

As you can see, we just introduced a new Node.js module, child_process. We did so because it allows us to make use of a very
simple yet useful non-blocking operation: exec().


42

What exec() does is, it executes a shell command from within
Node.js. In this example, we are going to use it to get a list of
all files in the current directory (“ls -lah”), allowing us to display
this list in the browser of a user requesting the /start URL.
What the code does is straightforward: create a new variable
content (with an initial value of “empty”), execute “ls -lah”, fill
the variable with the result, and return it.
As always, we will start our application, and visit http://localhost:8888/start¹⁵.
Which loads a beautiful web page that displays the string “empty”.
What’s going wrong here?
Well, as you may have already guessed, exec() does its magic in a
non-blocking fashion. That’s a good thing, because this way we
can execute very expensive shell operations (like, e.g., copying
huge files around or similar stuff) without forcing our application
into a full stop as the blocking sleep operation did.
(If you would like to prove this, replace “ls -lah” with a more
expensive operation like “find /”).
But we aren’t exactly happy with our elegant non-blocking
operation, when our browser doesn’t display its result, right?
Well, then, let’s fix it. And while we are at it, let’s try to
understand why the current architecture doesn’t work.
The problem is that exec(), in order to work non-blocking, makes
use of a callback function.
In our example, it’s an anonymous function which is passed as
the second parameter to the exec() function call:
¹⁵http://localhost:8888/start


43

function (error, stdout, stderr) {
content = stdout;
}

And herein lies the root of our problem: our own code is executed
synchronous, which means that immediately after calling exec(),
Node.js continues to execute return content;. At this point,
content is still “empty”, due to the fact that the callback function
passed to exec() has not yet been called - because exec() operates
asynchronous.
Now, “ls -lah” is a very inexpensive and fast operation (unless
there are millions of files in the directory). Which is why the
callback is called relatively expeditious - but it nevertheless
happens asynchronously.
Thinking about a more expensive command makes this more
obvious: “find /” takes about 1 minute on my machine, but if
I replace “ls -lah” with “find /” in the request handler, I still
immediately receive an HTTP response when opening the /start
URL - it’s clear that exec() does something in the background,
while Node.js itself continues with the application, and we may
assume that the callback function we passed into exec() will be
called only when the “find /” command has finished running.
But how can we achieve our goal, i.e. showing the user a list of
files in the current directory?
Well, after learning how to not do it, let’s discuss how to make
our request handlers respond to browser requests the right way.


44

Responding request handlers with
non-blocking operations
I’ve just used the phrase “the right way”. Dangerous stuff. Quite
often, there is no single “right way”.
But one possible solution for this is, as often with Node.js, to pass
functions around. Let’s examine this.
Right now, our application is able to transport the content (which
the request handlers would like to display to the user) from the
request handlers to the HTTP server by returning it up through
the layers of the application (request handler -> router -> server).
Our new approach is as follows: instead of bringing the content to the server, we will bring the server to the content. To
be more precise, we will inject the response object (from our
server’s callback function onRequest()) through the router into
the request handlers. The handlers will then be able to use this
object’s functions to respond to requests themselves.
Enough explanation, here is the step by step recipe on how to
change our application.
Let’s start with our server.js:


45

route(handle, pathname, response);
}
}

Instead of expecting a return value from the route() function,
we pass it a third parameter, our response object. Furthermore,
we removed any response method calls from the onRequest()
handler, because we now expect route to take care of that.
Next comes router.js:


46

function route(handle, pathname, response) {
handle[pathname](response);
} else {
response.write("404 Not found");
response.end();
}
}

Same pattern: instead of expecting a return value from our
request handlers, we pass the respond object on.
If no request handler can be used, we now take care of responding
with a proper “404” header and body ourselves.
And last but not least, we modify requestHandlers.js:
function start(response) {
exec("ls -lah", function (error, stdout, stderr) {
response.write(stdout);
response.end();
});


47

}
function upload(response) {
response.write("Hello Upload");
response.end();
}

Our handler functions need to accept the response parameter,
and have to make use of them in order to respond to the request
directly.
The start handler will respond from within the anonymous
exec() callback, and the upload handler still simply replies with
“Hello Upload”, but now by making use of the response object.
If we start our application again (node index.js), this should work
as expected.
If you would like to prove that an expensive operation behind
/start will no longer block requests for /upload from answering
immediately, then modify your requestHandlers.js as follows:


48

exec("find /",
{ timeout: 10000, maxBuffer: 20000*1024 },
function (error, stdout, stderr) {
response.write(stdout);
response.end();
});
}
response.end();
}

This will make HTTP requests to http://localhost:8888/start¹⁶
take at least 10 seconds, but requests to http://localhost:8888/upload¹⁷
will be answered immediately, even if /start is still computing.
¹⁶http://localhost:8888/start
¹⁷http://localhost:8888/upload


49

Serving something useful
Until now, what we have done is all fine and dandy, but we
haven’t created any value for the customers of our awardwinning website.
Our server, router, and request handlers are in place, thus now
we can begin to add content to our site which allows our users
to interact and walk through the use case of choosing a file,
uploading this file, and viewing the uploaded file in the browser.
For the sake of simplicity we will assume that only image files
are going to be uploaded and displayed through the application.
Ok, let’s take it step by step, but with most of the techniques and
principles of JavaScript explained by now, let’s at the same time
accelerate a bit. This author likes to hear himself talking way too
much anyways.
Here, step by step means roughly two steps: We will first look
at how to handle incoming POST requests (but not file uploads),
and in a second step, we will make use of an external Node.js
module for the file upload handling. I’ve chosen this approach
for two reasons.
First, handling basic POST requests is relatively simple with
Node.js, but still teaches us enough to be worth exercising it.
Second, handling file uploads (i.e., multipart POST requests) is
not simple with Node.js, and therefore is beyond the scope of
this tutorial, but using an external module is itself a lesson that
makes sense to be included in a beginner’s tutorial.


50

Handling POST requests
Let’s keep this banally simple: We will present a textarea that
can be filled by the user and submitted to the server in a
POST request. Upon receiving and handling this request, we will
display the content of the textarea.
The HTML for this textarea form needs to be served by our
/start request handler, so let’s add it right away, in file requestHandlers.js:
var body = '<html>'+
'<head>'+
'<meta http-equiv="Content-Type" content="text/html; '+
'charset=UTF-8" />'+
'</head>'+
'<body>'+
'<form action="/upload" method="post">'+
'<textarea name="text" rows="20" cols="60"></textarea>'+
'<input type="submit" value="Submit text" />'+
'</form>'+
'</body>'+
'</html>';
response.writeHead(200, {"Content-Type": "text/html"});
response.write(body);
response.end();
}


51

response.end();
}

Now if this isn’t going to win the Webby Awards, then I don’t
know what could. You should see this very simple form when
requesting http://localhost:8888/start¹⁸ in your browser. If not,
you probably didn’t restart the application.
I hear you: having view content right in the request handler
is ugly. However, I decided to not include that extra level of
abstraction (i.e., separating view and controller logic) in this
tutorial, because I think that it doesn’t teach us anything worth
knowing in the context of JavaScript or Node.js.
Let’s rather use the remaining screen space for a more interesting
problem, that is, handling the POST request that will hit our
/upload request handler when the user submits this form.
Now that we are becoming expert novices, we are no longer
surprised by the fact that handling POST data is done in a nonblocking fashion, by using asynchronous callbacks.
Which makes sense, because POST requests can potentially be
very large - nothing stops the user from entering text that is
¹⁸http://localhost:8888/start


52

multiple megabytes in size. Handling the whole bulk of data in
one go would result in a blocking operation.
To make the whole process non-blocking, Node.js serves our code
the POST data in small chunks, callbacks that are called upon
certain events. These events are data (an new chunk of POST
data arrives) and end (all chunks have been received).
We need to tell Node.js which functions to call back to when
these events occur. This is done by adding listeners to the request
object that is passed to our onRequest callback whenever an
HTTP request is received.
This basically looks like this:
request.addListener("data", function(chunk) {
// called when a new chunk of data was received
});
request.addListener("end", function() {
// called when all chunks of data have been received
});

The question arises where to implement this logic. We currently
can access the request object in our server only - we don’t pass
it on to the router and the request handlers, like we did with the
response object.
In my opinion, it’s an HTTP servers job to give the application
all the data from a requests it needs to do its job. Therefore, I
suggest we handle the POST data processing right in the server
and pass the final data on to the router and the request handlers,
which then can decide what to do with it.


53

Thus, the idea is to put the data and end event callbacks in the
server, collecting all POST data chunks in the data callback, and
calling the router upon receiving the end event, while passing
the collected data chunks on to the router, which in turn passes
it on to the request handlers.
Here we go, starting with server.js:
var postData = "";
request.setEncoding("utf8");
request.addListener("data", function(postDataChunk) {
postData += postDataChunk;
console.log("Received POST data chunk '"+
postDataChunk + "'.");
});
request.addListener("end", function() {
route(handle, pathname, response, postData);
});
}


54

}

We basically did three things here: First, we defined that we
expect the encoding of the received data to be UTF-8, we added
an event listener for the “data” event which step by step fills
our new postData variable whenever a new chunk of POST data
arrives, and we moved the call to our router into the end event
callback to make sure it’s only called when all POST data is
gathered. We also pass the POST data into the router, because
we are going to need it in our request handlers.
Adding the console logging on every chunk that is received
probably is a bad idea for production code (megabytes of POST
data, remember?), but makes sense to see what happens.
I suggest playing around with this a bit. Put small amounts of
text into the textarea as well as lots of text, and you will see that
for the larger texts, the data callback is indeed called multiple
times.
Let’s add even more awesome to our app. On the /upload page,
we will display the received content. To make this possible, we
need to pass the postData on to the request handlers, in router.js:


55

function route(handle, pathname, response, postData) {
handle[pathname](response, postData);
} else {
response.end();
}
}

And in requestHandlers.js, we include the data in our response
of the upload request handler:
function start(response, postData) {
'<head>'+
'</head>'+
'<body>'+
'</form>'+
'</body>'+


56

'</html>';
response.end();
}
function upload(response, postData) {
response.write("You've sent: " + postData);
response.end();
}

That’s it, we are now able to receive POST data and use it in our
request handlers.
One last thing for this topic: what we pass on to the router and the
request handlers is the complete body of our POST request. We
will probably want to consume the individual fields that make
up the POST data, in this case, the value of the text field.
We already read about the querystring module, which assists us
with this:


57

var querystring = require("querystring");
'<head>'+
'</head>'+
'<body>'+
'</form>'+
'</body>'+
'</html>';
response.end();
}
response.write("You've sent the text: "+
querystring.parse(postData).text);
response.end();
}


58


Well, for a beginner’s tutorial, that’s all there is to say about
handling POST data.

Handling file uploads
Let’s tackle our final use case. Our plan was to allow users to
upload an image file, and display the uploaded image in the
browser.
Back in the 90’s this would have qualified as a business model
for an IPO, today it must suffice to teach us two things: how to
install external Node.js libraries, and how to make use of them
in our own code.
The external module we are going to use is node-formidable by
Felix Geisendoerfer. It nicely abstracts away all the nasty details
of parsing incoming file data. At the end of the day, handling
incoming files is “only” about handling POST data - but the devil
really is in the details here, and using a ready-made solution
makes a lot of sense in this case.
In order to make use of Felix’ code, the according Node.js
module needs to be installed. Node.js ships with its own package
manager, dubbed NPM. It allows us to install external Node.js
modules in a very convenient fashion. Given a working Node.js
installation, it boils down to issuing


59

npm install formidable

on our command line. If the following output ends with
npm info build Success: formidable@1.0.2
npm ok

then we are good to go.
The formidable module is now available to our own code - all
we need to do is requiring it just like one of the built-in modules
we used earlier:
var formidable = require("formidable");

The metaphor formidable uses is that of a form being submitted
via HTTP POST, making it parseable in Node.js. All we need
to do is create a new IncomingForm, which is an abstraction of
this submitted form, and which can then be used to parse the
request object of our HTTP server for the fields and files that
were submitted through this form.
The example code from the node-formidable project page shows
how the different parts play together:


60

var formidable = require('formidable'),
http = require('http'),
sys = require('sys');
http.createServer(function(req, res) {
if (req.url == '/upload' && req.method.toLowerCase() == 'post') {
// parse a file upload
var form = new formidable.IncomingForm();
form.parse(req, function(error, fields, files) {
res.writeHead(200, {'content-type': 'text/plain'});
res.write('received upload:nn');
res.end(sys.inspect({fields: fields, files: files}));
});
return;
}
// show a file upload form
res.writeHead(200, {'content-type': 'text/html'});
res.end(
'<form action="/upload" enctype="multipart/form-data" '+
'method="post">'+
'<input type="text" name="title"><br>'+
'<input type="file" name="upload" multiple="multiple"><br>'+
'<input type="submit" value="Upload">'+
'</form>'
);
}).listen(8888);

If we put this code into a file and execute it through node, we
are able to submit a simple form, including a file upload, and see
how the files object, which is passed to the callback defined in


61

the form.parse call, is structured:
received upload:
{ fields: { title: 'Hello World' },
files:
{ upload:
{ size: 1558,
path: '/tmp/1c747974a27a6292743669e91f29350b',
name: 'us-flag.png',
type: 'image/png',
lastModifiedDate: Tue, 21 Jun 2011 07:02:41 GMT,
_writeStream: [Object],
length: [Getter],
filename: [Getter],
mime: [Getter] } } }

In order to make our use case happen, what we need to do is
to include the form-parsing logic of formidable into our code
structure, plus we will need to find out how to serve the content
of the uploaded file (which is saved into the /tmp folder) to a
requesting browser.
Let’s tackle the latter one first: if there is an image file on our
local hardrive, how do we serve it to a requesting browser?
We are obviously going to read the contents of this file into our
Node.js server, and unsurprisingly, there is a module for that it’s called fs.
Let’s add another request handler for the URL /show, which will
hardcodingly display the contents of the file /tmp/test.png. It of


62

course makes a lot of sense to save a real png image file to this
location first.
We are going to modify requestHandlers.js as follows:
var querystring = require("querystring"),
fs = require("fs");
'<head>'+
'<meta http-equiv="Content-Type" '+
'content="text/html; charset=UTF-8" />'+
'</head>'+
'<body>'+
'</form>'+
'</body>'+
'</html>';
response.end();
}


63

response.end();
}
function show(response, postData) {
console.log("Request handler 'show' was called.");
fs.readFile("/tmp/test.png", "binary", function(error, file) {
if (error) {
response.write(error + "n");
response.end();
} else {
response.writeHead(200, {"Content-Type": "image/png"});
response.write(file, "binary");
response.end();
}
});
}
exports.show = show;

We also need to map this new request handler to the URL /show
in file index.js:


64

var requestHandlers = require("./requestHandlers");
var handle = {}
handle["/"] = requestHandlers.start;
handle["/start"] = requestHandlers.start;
handle["/upload"] = requestHandlers.upload;
handle["/show"] = requestHandlers.show;
server.start(router.route, handle);

By restarting the server and opening http://localhost:8888/show¹⁹
in the browser, the image file saved at /tmp/test.png should be
displayed.
Fine. All we need to do now is
• add a file upload element to the form which is served at
/start,
• integrate node-formidable into the upload request handler, in order to save the uploaded file to /tmp/test.png,
• embed the uploaded image into the HTML output of the
/upload URL.
Step 1 is simple. We need to add an encoding type of multipart/formdata to our HTML form, remove the textarea, add a file upload
input field, and change the submit button text to “Upload file”.
Let’s do just that in file requestHandlers.js:
¹⁹http://localhost:8888/show


65

fs = require("fs");
'<head>'+
'</head>'+
'<body>'+
'method="post">'+
'<input type="file" name="upload">'+
'<input type="submit" value="Upload file" />'+
'</form>'+
'</body>'+
'</html>';
response.end();
}
response.end();


66

}
function show(response, postData) {
if (error) {
response.end();
} else {
response.end();
}
});
}

Great. The next step is a bit more complex of course. The first
problem is: we want to handle the file upload in our upload
request handler, and there, we will need to pass the request
object to the form.parse call of node-formidable.
But all we have is the response object and the postData array. Sad
panda. Looks like we will have to pass the request object all the
way from the server to the router to the request handler. There
may be more elegant solutions, but this approach should do the
job for now.


67

And while we are at it, let’s remove the whole postData stuff
in our server and request handlers - we won’t need it for
handling the file upload, and it even raises a problem: we already
“consumed” the data events of the request object in the server,
which means that form.parse, which also needs to consume those
events, wouldn’t receive any more data from them (because
Node.js doesn’t buffer any data).
Let’s start with server.js - we remove the postData handling
and the request.setEncoding line (which is going to be handled
by node-formidable itself), and we pass request to the router
instead:
route(handle, pathname, response, request);
}
}

Next comes router.js - we don’t need to pass postData on
anymore, and instead pass request:


68

function route(handle, pathname, response, request) {
handle[pathname](response, request);
} else {
response.end();
}
}

Now, the request object can be used in our upload request
handler function. node-formidable will handle the details of
saving the uploaded file to a local file within /tmp, but we need
to make sure that this file is renamed to /tmp/test.png ourselves.
Yes, we keep things really simple and assume that only PNG
images will be uploaded.
There is a bit of extra-complexity in the rename logic: the
Windows implementation of node doesn’t like it when you try to
rename a file onto the position of an existing file, which is why
we need to delete the file in case of an error.
Let’s put the pieces of managing the uploaded file and renaming
it together now, in file requestHandlers.js:


69

fs = require("fs"),
formidable = require("formidable");
'<head>'+
'</head>'+
'<body>'+
'method="post">'+
'<input type="file" name="upload" multiple="multiple">'+
'<input type="submit" value="Upload file" />'+
'</form>'+
'</body>'+
'</html>';
response.end();
}
function upload(response, request) {
var form = new formidable.IncomingForm();
console.log("about to parse");


70

form.parse(request, function(error, fields, files) {
console.log("parsing done");
/* Possible error on Windows systems:
tried to rename to an already existing file */
fs.rename(files.upload.path, "/tmp/test.png", function(error) {
if (error) {
fs.unlink("/tmp/test.png");
fs.rename(files.upload.path, "/tmp/test.png");
}
});
response.write("received image:<br/>");
response.write("<img src='/show' />");
response.end();
});
}
function show(response) {
if(error) {
response.end();
} else {
response.end();
}
});


71

}

And that’s it. Restart the server, and the complete use case will be
available. Select a local PNG image from your hardrive, upload
it to the server, and have it displayed in the web page.

Conclusion and outlook
Congratulations, our mission is accomplished! We wrote a simple yet full-fledged Node.js web application. We talked about
server-side JavaScript, functional programming, blocking and
non-blocking operations, callbacks, events, custom, internal and
external modules, and a lot more.
Of course, there’s a lot of stuff we did not talk about: how to
talk to a database, how to write unit tests, how to create external
modules that are installable via NPM, or even something simple
like how to handle GET requests.
But that’s the fate of every book aimed at beginners - it can’t talk
about every single aspect in every single detail.
The good news is, the Node.js community is extremly vibrant
(think of an ADHD kid on caffeine, but in a positive way), which
means there are a lot of resources out there, and a lot of places to
get your questions answered. The Node.js community wiki²⁰ and
the NodeCloud directory²¹ are probably the best starting points
for more information.

²⁰https://github.com/joyent/node/wiki
²¹http://www.nodecloud.org/

72

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