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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
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<div>
<h1>The Node Beginner Book</h1>
<div id="author">A Node.js tutorial by <a href="http://twitter.com/manuelkiessling">Manuel Kiessling</a></div>
<a name="about"></a>
<h2>About</h2>
<p>
The aim of this document is to get you started with developing
applications with Node.js, teaching you everything you need to
know about "advanced" JavaScript along the way. It goes way
beyond your typical "Hello World" tutorial.
</p>
<a name="status"></a>
<h3>Status</h3>
<p>
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 August 27, 2014.
</p>
<p>
The code samples in this book are tested to work with Node.js
version 0.10.29.
</p>
<p>
This site allows you to read pages the first 20 pages of this book
for free.
The complete text is available as a DRM-free eBook (PDF, ePub and
Kindle format). More info is available <a href="#salespitch">at the
end of the free part</a>.
</p>
<a name="intended-audience"></a>
<h3>Intended audience</h3>
<p>
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.
</p>
<p>
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.
</p>
<p>
However, because functions and objects in JavaScript are different
from their counterparts in most other languages, these will be
explained in more detail.
</p>
<a name="structure"></a>
<h3>Structure of this document</h3>
<p>
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.
</p>
<p>
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.
</p>
<p>
We will start with looking at how JavaScript development in Node.js
is different from JavaScript development in a browser.
</p>
<p>
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.
</p>
<p>
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.
</p>
<p>
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.
</p>
<p>
The source code of the finished application is available through
<a href="https://github.com/ManuelKiessling/NodeBeginnerBook/tree/master/code/application">the
NodeBeginnerBook Github repository</a>.
</p>
<div id="table-of-contents-headline">Table of contents</div>
<div id="table-of-contents">
<ul>
<li><a href="#about">About</a>
<ul>
<li><a href="#status">Status</a></li>
<li><a href="#intended-audience">Intended audience</a></li>
<li><a href="#structure">Structure of this document</a></li>
</ul>
</li>
<li><a href="#javascript-and-nodejs">JavaScript and Node.js</a>
<ul>
<li><a href="#javascript-and-you">JavaScript and You</a></li>
<li><a href="#a-word-of-warning">A word of warning</a></li>
<li><a href="#server-side-javascript">Server-side JavaScript</a></li>
<li><a href="#hello-world">"Hello World"</a></li>
</ul>
</li>
<li><a href="#a-full-blown-web-application-with-nodejs">A full blown web application with Node.js</a>
<ul>
<li><a href="#the-use-cases">The use cases</a></li>
<li><a href="#the-application-stack">The application stack</a></li>
</ul>
</li>
<li><a href="#building-the-application-stack">Building the application stack</a>
<ul>
<li><a href="#a-basic-http-server">A basic HTTP server</a></li>
<li><a href="#analyzing-our-http-server">Analyzing our HTTP server</a></li>
<li><a href="#passing-functions-around">Passing functions around</a></li>
<li><a href="#how-function-passing-makes-our-http-server-work">How function passing makes our
HTTP server work</a></li>
<li><a href="#event-driven-callbacks">Event-driven asynchronous callbacks</a></li>
<li><a href="#how-our-server-handles-requests">How our server handles requests</a></li>
<li><a href="#finding-a-place-for-our-server-module">Finding a place for our server module</a>
</li>
<li><a href="#whats-needed-to-route-requests">What's needed to "route" requests?</a></li>
<li><a href="#execution-in-the-kongdom-of-verbs">Execution in the kingdom of verbs</a></li>
<li><a href="#routing-to-real-request-handlers">Routing to real request handlers</a></li>
<li><br/><strong>Chapters available in the full book:</strong><br/><br/>
<li>Making the request handlers respond
<ul>
<li>How to not do it</li>
<li>Blocking and non-blocking</li>
<li>Responding request handlers with non-blocking operation</li>
</ul>
</li>
<li>Serving something useful
<ul>
<li>Handling POST requests</li>
<li>Handling file uploads</li>
</ul>
</li>
<li>Conclusion and outlook</li>
</li>
</ul>
</li>
</ul>
</div>
<a name="javascript-and-nodejs"></a>
<h2>JavaScript and Node.js</h2>
<a name="javascript-and-you"></a>
<h3>JavaScript and You</h3>
<p>
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.
</p>
<p>
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.
</p>
<p>
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.
</p>
<p>
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 <em>window.open()</em>.
</p>
<p>
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 <em>user</em>, but not a JavaScript
<em>developer</em>.
</p>
<p>
And then came Node.js. JavaScript on the server, how cool
is that?
</p>
<p>
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.
</p>
<p>
Here is the problem: Because JavaScript really lives two,
maybe even three lives (the funny little DHTML helper from
the mid-90'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.
</p>
<p>
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.
</p>
<p>
There is, of course, excellent documentation out there.
But documentation alone sometimes isn't enough. What is
needed is guidance.
</p>
<p>
My goal is to provide a guide for you.
</p>
<a name="a-word-of-warning"></a>
<h3>A word of warning</h3>
<p>
There are some really excellent JavaScript people out
there. I'm not one of them.
</p>
<p>
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 learned some of the more
advanced aspects of JavaScript just recently.
I'm not experienced.
</p>
<p>
Which is why this is no "from novice to expert" book. It's
more like "from novice to advanced novice".
</p>
<p>
If I don't fail, then this will be the kind of
document I wish I had when starting with Node.js.
</p>
<a name="server-side-javascript"></a>
<h3>Server-side JavaScript</h3>
<p>
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.
</p>
<p>
Node.js really is just another context: it allows you to run
JavaScript code in the backend, outside a browser.
</p>
<p>
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.
</p>
<p>
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.
</p>
<p>
Thus, Node.js is really two things: a runtime environment and a
library.
</p>
<p>
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
<a href="https://github.com/joyent/node/wiki/Installation" title="Building and Installing Node.js">the
official
installation instructions</a>. Please come back once you
are up and running.
</p>
<a name="hello-world"></a>
<h3>"Hello World"</h3>
<p>
Ok, let's just jump in the cold water and write our first
Node.js application: "Hello World".
</p>
<p>
Open your favorite editor and create a file called
<em>helloworld.js</em>. We want it to write "Hello World"
to STDOUT, and here is the code needed to do that:
</p>
<pre class="prettyprint lang-js"><span class="pln">console</span><span class="pun">.</span><span
class="pln">log</span><span class="pun">(</span><span class="str">"Hello World"</span><span class="pun">);</span></pre>
<p>
Save the file, and execute it through Node.js:
</p>
<pre>node helloworld.js</pre>
<p>
This should output <em>Hello World</em> on your terminal.
</p>
<p>
Ok, this stuff is boring, right? Let's write some real
stuff.
</p>
<a name="a-full-blown-web-application-with-nodejs"></a>
<h2>A full blown web application with Node.js</h2>
<a name="the-use-cases"></a>
<h3>The use cases</h3>
<p>
Let's keep it simple, but realistic:
</p>
<ul>
<li>
The user should be able to use our application with
a web browser
</li>
<li>
The user should see a welcome page when
requesting http://<em>domain</em>/start which displays a
file upload form
</li>
<li>
By choosing an image file to upload and submitting the
form, this image should then be uploaded to
http://<em>domain</em>/upload, where it is displayed once
the upload is finished
</li>
</ul>
<p>
Fair enough. Now, you could achieve this goal by googling
and hacking together <em>something</em>. But that's not
what we want to do here.
</p>
<p>
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.
</p>
<a name="the-application-stack"></a>
<h3>The application stack</h3>
<p>
Let's dissect our application. Which parts need to be
implemented in order to fulfill the use cases?
</p>
<ul>
<li>
We want to serve web pages, therefore we need an
<strong>HTTP server</strong>
</li>
<li>
Our server will need to answer differently to
requests, depending on which URL the request was
asking for, thus we need some kind of
<strong>router</strong> in order to map requests
to request handlers
</li>
<li>
To fullfill the requests that arrived at the server
and have been routed using the router, we need
actual <strong>request handlers</strong>
</li>
<li>
The router probably should also treat any incoming
POST data and give it to the request handlers in
a convenient form, thus we need <strong>request
data handling</strong>
</li>
<li>
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
<strong>view logic</strong> the request handlers
can use in order to send content to the user's
browser
</li>
<li>
Last but not least, the user will be able to upload
images, so we are going to need some kind of
<strong>upload handling</strong> which takes care of
the details
</li>
</ul>
<p>
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.
<br>
Which in turn means that the whole "we need to be able to
serve web pages and receive HTTP requests" stuff doesn't
happen within PHP itself.
</p>
<p>
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.
</p>
<p>
This might sound like a lot of work, but we will see in a
moment that with Node.js, it's not.
</p>
<p>
Let's just start at the beginning and implement the first
part of our stack, the HTTP server.
</p>
<a name="building-the-application-stack"></a>
<h2>Building the application stack</h2>
<a name="a-basic-http-server"></a>
<h3>A basic HTTP server</h3>
<p>
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.
<br>
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?
</p>
<p>
Turns out, it's relatively easy to keep the different
concerns of your code separated, by putting them in
modules.
</p>
<p>
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.
</p>
<p>
So, let's create a main file which we use to start our
application, and a module file where our HTTP server code
lives.
</p>
<p>
My impression is that it's more or less a standard to name
your main file <em>index.js</em>. It makes sense to put our
server module into a file named <em>server.js</em>.
</p>
<p>
Let's start with the server module. Create the file
<em>server.js</em> in the root directory of your project,
and fill it with the following code:
</p>
<pre class="prettyprint lang-js"><span class="kwd">var</span><span class="pln"> http </span><span
class="pun">=</span><span class="pln"> require</span><span class="pun">(</span><span
class="str">"http"</span><span class="pun">);</span><span class="pln"><br><br>http</span><span
class="pun">.</span><span class="pln">createServer</span><span class="pun">(</span><span class="kwd">function</span><span
class="pun">(</span><span class="pln">request</span><span class="pun">,</span><span class="pln"> response</span><span
class="pun">)</span><span class="pln"> </span><span class="pun">{</span><span class="pln"><br> response</span><span
class="pun">.</span><span class="pln">writeHead</span><span class="pun">(</span><span
class="lit">200</span><span class="pun">,</span><span class="pln"> </span><span
class="pun">{</span><span class="str">"Content-Type"</span><span class="pun">:</span><span
class="pln"> </span><span class="str">"text/plain"</span><span class="pun">});</span><span
class="pln"><br> response</span><span class="pun">.</span><span class="pln">write</span><span
class="pun">(</span><span class="str">"Hello World"</span><span class="pun">);</span><span
class="pln"><br> response</span><span class="pun">.</span><span class="pln">end</span><span
class="pun">();</span><span class="pln"><br></span><span class="pun">}).</span><span
class="pln">listen</span><span class="pun">(</span><span class="lit">8888</span><span
class="pun">);</span></pre>
<p>
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:
</p>
<pre>node server.js</pre>
<p>
Now, open your browser and point it at
<a href="http://localhost:8888/" rel="nofollow">http://localhost:8888/</a>.
This should display a web page that says "Hello World".
</p>
<p>
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.
</p>
<a name="analyzing-our-http-server"></a>
<h3>Analyzing our HTTP server</h3>
<p>
Well, then, let's analyze what's actually going on here.
</p>
<p>
The first line <em>require</em>s the <em>http</em> module
that ships with Node.js and makes it accessible through the
variable <em>http</em>.
</p>
<p>
We then call one of the functions the http module offers:
<em>createServer</em>. This function returns an object, and
this object has a method named <em>listen</em>, and takes
a numeric value which indicates the port number our HTTP
server is going to listen on.
</p>
<p>
Please ignore for a second the function definition that
follows the opening bracket of <em>http.createServer</em>.
</p>
<p>
We could have written the code that starts our server and
makes it listen at port 8888 like this:
</p>
<pre class="prettyprint lang-js"><span class="kwd">var</span><span class="pln"> http </span><span
class="pun">=</span><span class="pln"> require</span><span class="pun">(</span><span
class="str">"http"</span><span class="pun">);</span><span class="pln"><br><br></span><span class="kwd">var</span><span
class="pln"> server </span><span class="pun">=</span><span class="pln"> http</span><span
class="pun">.</span><span class="pln">createServer</span><span class="pun">();</span><span
class="pln"><br>server</span><span class="pun">.</span><span class="pln">listen</span><span class="pun">(</span><span
class="lit">8888</span><span class="pun">);</span></pre>
<p>
That would start an HTTP server listening at port 8888
and doing nothing else (not even answering any incoming
requests).
</p>
<p>
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 <em>createServer()</em> call.
</p>
<p>
Turns out, this function definition IS the first (and only)
parameter we are giving to the <em>createServer()</em>
call. Because in JavaScript, functions can be passed around
like any other value.
</p>
<a name="passing-functions-around"></a>
<h3>Passing functions around</h3>
<p>
You can, for example, do something like this:
</p>
<pre class="prettyprint lang-js"><span class="kwd">function</span><span class="pln"> say</span><span
class="pun">(</span><span class="pln">word</span><span class="pun">)</span><span
class="pln"> </span><span class="pun">{</span><span class="pln"><br> console</span><span
class="pun">.</span><span class="pln">log</span><span class="pun">(</span><span
class="pln">word</span><span class="pun">);</span><span class="pln"><br></span><span
class="pun">}</span><span class="pln"><br><br></span><span class="kwd">function</span><span class="pln"> execute</span><span
class="pun">(</span><span class="pln">someFunction</span><span class="pun">,</span><span class="pln"> value</span><span
class="pun">)</span><span class="pln"> </span><span class="pun">{</span><span class="pln"><br> someFunction</span><span
class="pun">(</span><span class="pln">value</span><span class="pun">);</span><span
class="pln"><br></span><span class="pun">}</span><span class="pln"><br><br>execute</span><span
class="pun">(</span><span class="pln">say</span><span class="pun">,</span><span
class="pln"> </span><span class="str">"Hello"</span><span class="pun">);</span></pre>
<p>
Read this carefully! What we are doing here is, we pass the
function <em>say</em> as the first parameter to the
<em>execute</em> function. Not the return value of
<em>say</em>, but <em>say</em> itself!
</p>
<p>
Thus, <em>say</em> becomes the local variable
<em>someFunction</em> within <em>execute</em>, and execute
can call the function in this variable by issuing
<em>someFunction()</em> (adding brackets).
</p>
<p>
Of course, because <em>say</em> takes one parameter,
<em>execute</em> can pass such a parameter when calling
<em>someFunction</em>.
</p>
<p>
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:
</p>
<pre class="prettyprint lang-js"><span class="kwd">function</span><span class="pln"> execute</span><span
class="pun">(</span><span class="pln">someFunction</span><span class="pun">,</span><span class="pln"> value</span><span
class="pun">)</span><span class="pln"> </span><span class="pun">{</span><span class="pln"><br> someFunction</span><span
class="pun">(</span><span class="pln">value</span><span class="pun">);</span><span
class="pln"><br></span><span class="pun">}</span><span class="pln"><br><br>execute</span><span
class="pun">(</span><span class="kwd">function</span><span class="pun">(</span><span
class="pln">word</span><span class="pun">){</span><span class="pln"> console</span><span
class="pun">.</span><span class="pln">log</span><span class="pun">(</span><span
class="pln">word</span><span class="pun">)</span><span class="pln"> </span><span
class="pun">},</span><span class="pln"> </span><span class="str">"Hello"</span><span
class="pun">);</span></pre>
<p>
We define the function we want to pass to <em>execute</em>
right there at the place where <em>execute</em> expects its
first parameter.
</p>
<p>
This way, we don't even need to give the function a name,
which is why this is called an <em>anonymous function</em>.
</p>
<p>
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.
</p>
<a name="how-function-passing-makes-our-http-server-work"></a>
<h3>How function passing makes our HTTP server work</h3>
<p>
With this knowledge, let's get back to our minimalistic
HTTP server:
</p>
<pre class="prettyprint lang-js"><span class="kwd">var</span><span class="pln"> http </span><span
class="pun">=</span><span class="pln"> require</span><span class="pun">(</span><span
class="str">"http"</span><span class="pun">);</span><span class="pln"><br><br>http</span><span
class="pun">.</span><span class="pln">createServer</span><span class="pun">(</span><span class="kwd">function</span><span
class="pun">(</span><span class="pln">request</span><span class="pun">,</span><span class="pln"> response</span><span
class="pun">)</span><span class="pln"> </span><span class="pun">{</span><span class="pln"><br> response</span><span
class="pun">.</span><span class="pln">writeHead</span><span class="pun">(</span><span
class="lit">200</span><span class="pun">,</span><span class="pln"> </span><span
class="pun">{</span><span class="str">"Content-Type"</span><span class="pun">:</span><span
class="pln"> </span><span class="str">"text/plain"</span><span class="pun">});</span><span
class="pln"><br> response</span><span class="pun">.</span><span class="pln">write</span><span
class="pun">(</span><span class="str">"Hello World"</span><span class="pun">);</span><span
class="pln"><br> response</span><span class="pun">.</span><span class="pln">end</span><span
class="pun">();</span><span class="pln"><br></span><span class="pun">}).</span><span
class="pln">listen</span><span class="pun">(</span><span class="lit">8888</span><span
class="pun">);</span></pre>
<p>
By now it should be clear what we are actually doing here:
we pass the <em>createServer</em> function an anonymous
function.
</p>
<p>
We could achieve the same by refactoring our code to:
</p>
<pre class="prettyprint lang-js"><span class="kwd">var</span><span class="pln"> http </span><span
class="pun">=</span><span class="pln"> require</span><span class="pun">(</span><span
class="str">"http"</span><span class="pun">);</span><span class="pln"><br><br></span><span class="kwd">function</span><span
class="pln"> onRequest</span><span class="pun">(</span><span class="pln">request</span><span
class="pun">,</span><span class="pln"> response</span><span class="pun">)</span><span
class="pln"> </span><span class="pun">{</span><span class="pln"><br> response</span><span
class="pun">.</span><span class="pln">writeHead</span><span class="pun">(</span><span
class="lit">200</span><span class="pun">,</span><span class="pln"> </span><span
class="pun">{</span><span class="str">"Content-Type"</span><span class="pun">:</span><span
class="pln"> </span><span class="str">"text/plain"</span><span class="pun">});</span><span
class="pln"><br> response</span><span class="pun">.</span><span class="pln">write</span><span
class="pun">(</span><span class="str">"Hello World"</span><span class="pun">);</span><span
class="pln"><br> response</span><span class="pun">.</span><span class="pln">end</span><span
class="pun">();</span><span class="pln"><br></span><span class="pun">}</span><span class="pln"><br><br>http</span><span
class="pun">.</span><span class="pln">createServer</span><span class="pun">(</span><span class="pln">onRequest</span><span
class="pun">).</span><span class="pln">listen</span><span class="pun">(</span><span
class="lit">8888</span><span class="pun">);</span></pre>
<p>
Maybe now is a good moment to ask: Why are we doing it
that way?
</p>
<a name="event-driven-callbacks"></a>
<h3>Event-driven asynchronous callbacks</h3>
<p>
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.
</p>
<p>
Let's take a very simple piece of code like this:
</p>
<pre class="prettyprint lang-js">var result = database.query("SELECT * FROM hugetable");
console.log("Hello World");</pre>
<p>
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.
</p>
<p>
Let's assume that the database query is really slow, that it has
to read an awful lot of rows, which takes several seconds.
</p>
<p>
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 <em>console.log()</em>
function.
</p>
<p>
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.
</p>
<p>
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.
</p>
<p>
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.
</p>
<p>
To avoid this, JavaScript, and therefore Node.js, introduces the
concept of event-driven, asynchronous callbacks, by utilizing an
event loop.
</p>
<p>
We can understand this concept by analyzing a rewritten version
of our problematic code:
</p>
<pre class="prettyprint lang-js">database.query("SELECT * FROM hugetable", function(rows) {
var result = rows;
});
console.log("Hello World");</pre>
<p>
Here, instead of expecting <em>database.query()</em> to directly
return a result to us, we pass it a second parameter, an anonymous
function.
</p>
<p>
In its previous form, our code was synchronous: <em>first</em>
do the database query, and only when this is done, <em>then</em>
write to the console.
</p>
<p>
Now, Node.js can handle the database request asynchronously.
Provided that <em>database.query()</em> 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 <em>database.query()</em>."
</p>
<p>
Then, it immediately executes <em>console.log()</em>, 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.