The Node Beginner Book
About
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.
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 September 9, 2012.
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.
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.
The source code of the finished application is available through
the
NodeBeginnerBook Github repository.
Table of contents
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?
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 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.
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 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.
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 instructions. 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.
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.
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 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:
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/.
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 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 http = require("http");
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:
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");
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:
var http = require("http");
http.createServer(function(request, response) {
response.writeHead(200, {"Content-Type": "text/plain"});
response.write("Hello World");
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:
var http = require("http");
function onRequest(request, response) {
response.writeHead(200, {"Content-Type": "text/plain"});
response.write("Hello World");
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");
console.log("Hello World");
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.
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:
database.query("SELECT * FROM hugetable", function(rows) {
var result = rows;
});
console.log("Hello World");
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 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
Geisendörfer's excellent post
Understanding
node.js
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:
var http = require("http");
function onRequest(request, response) {
console.log("Request received.");
response.writeHead(200, {"Content-Type": "text/plain"});
response.write("Hello World");
response.end();
}
http.createServer(onRequest).listen(8888);
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/
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 browsers will try to load the favicon by requesting
http://localhost:8888/favicon.ico whenever you open
http://localhost:8888/).
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).
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:
var http = require("http");
...
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?
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:
var http = require("http");
function start() {
function onRequest(request, response) {
console.log("Request received.");
response.writeHead(200, {"Content-Type": "text/plain"});
response.write("Hello World");
response.end();
}
http.createServer(onRequest).listen(8888);
console.log("Server has started.");
}
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.
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.
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:
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 http = require("http");
var url = require("url");
function start() {
function onRequest(request, response) {
var pathname = url.parse(request.url).pathname;
console.log("Request for " + pathname + " received.");
response.writeHead(200, {"Content-Type": "text/plain"});
response.write("Hello World");
response.end();
}
http.createServer(onRequest).listen(8888);
console.log("Server has started.");
}
exports.start = start;
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, 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 Injection
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:
var http = require("http");
var url = require("url");
function start(route) {
function onRequest(request, response) {
var pathname = url.parse(request.url).pathname;
console.log("Request for " + pathname + " received.");
route(pathname);
response.writeHead(200, {"Content-Type": "text/plain"});
response.write("Hello World");
response.end();
}
http.createServer(onRequest).listen(8888);
console.log("Server has started.");
}
exports.start = start;
And let's extend our index.js accordingly, that is,
injecting the route function of our router into the server:
var server = require("./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 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 Nouns.
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.
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.start = start;
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 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/en-us/magazine/cc163419.aspx,
let me quote the relevant part:
In C++ or C#, when we’re talking about objects, we're
referring to instances of classes or structs. 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 server = require("./server");
var router = require("./router");
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.
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:
var http = require("http");
var url = require("url");
function start(route, handle) {
function onRequest(request, response) {
var pathname = url.parse(request.url).pathname;
console.log("Request for " + pathname + " received.");
route(handle, pathname);
response.writeHead(200, {"Content-Type": "text/plain"});
response.write("Hello World");
response.end();
}
http.createServer(onRequest).listen(8888);
console.log("Server has started.");
}
exports.start = start;
We've added the handle parameter to our
start() function, and 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) {
console.log("About to route a request for " + pathname);
if (typeof handle[pathname] === 'function') {
handle[pathname]();
} else {
console.log("No request handler found for " + pathname);
}
}
exports.route = route;
What we do here is, we check if a request handler for the given
pathname exists, and if it does, we simply call the corresponding
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/start
in our browser, we can prove that the correct request handler
was indeed called:
Server has started.
Request for /start received.
About to route a request for /start
Request handler 'start' was called.
And opening http://localhost:8888/
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 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() {
console.log("Request handler 'start' was called.");
return "Hello Start";
}
function upload() {
console.log("Request handler 'upload' was called.");
return "Hello Upload";
}
exports.start = start;
exports.upload = 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:
function route(handle, pathname) {
console.log("About to route a request for " + pathname);
if (typeof handle[pathname] === 'function') {
return handle[pathname]();
} else {
console.log("No request handler found for " + pathname);
return "404 Not found";
}
}
exports.route = route;
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 http = require("http");
var url = require("url");
function start(route, handle) {
function onRequest(request, response) {
var pathname = url.parse(request.url).pathname;
console.log("Request for " + pathname + " received.");
response.writeHead(200, {"Content-Type": "text/plain"});
var content = route(handle, pathname)
response.write(content);
response.end();
}
http.createServer(onRequest).listen(8888);
console.log("Server has started.");
}
exports.start = start;
If we start our rewritten application, everything works like
a charm: requesting http://localhost:8888/start
results in "Hello Start" being displayed in the browser,
requesting http://localhost:8888/upload
gives us "Hello Upload", and http://localhost:8888/foo
produces "404 Not found".
Ok, then why is that a problem? The short answer: because we
will run into problems if one of 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
"non-blocking" means, let's demonstrate ourselves what happens
if we add a blocking operation to our request handlers.
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() {
console.log("Request handler 'start' was called.");
function sleep(milliSeconds) {
var startTime = new Date().getTime();
while (new Date().getTime() < startTime + milliSeconds);
}
sleep(10000);
return "Hello Start";
}
function upload() {
console.log("Request handler 'upload' was called.");
return "Hello Upload";
}
exports.start = start;
exports.upload = 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.
(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/start
into the address bar, but do not yet open this url!
In the second browser window's address bar, enter http://localhost:8888/upload,
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 corrsponding 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
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() {
console.log("Request handler 'start' was called.");
var content = "empty";
exec("ls -lah", function (error, stdout, stderr) {
content = stdout;
});
return content;
}
function upload() {
console.log("Request handler 'upload' was called.");
return "Hello Upload";
}
exports.start = start;
exports.upload = 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().
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/start.
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:
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.
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:
var http = require("http");
var url = require("url");
function start(route, handle) {
function onRequest(request, response) {
var pathname = url.parse(request.url).pathname;
console.log("Request for " + pathname + " received.");
route(handle, pathname, response);
}
http.createServer(onRequest).listen(8888);
console.log("Server has started.");
}
exports.start = start;
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:
function route(handle, pathname, response) {
console.log("About to route a request for " + pathname);
if (typeof handle[pathname] === 'function') {
handle[pathname](response);
} else {
console.log("No request handler found for " + pathname);
response.writeHead(404, {"Content-Type": "text/plain"});
response.write("404 Not found");
response.end();
}
}
exports.route = route;
Same pattern: instead of expecting a return value from our
request handlers, we pass the response 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:
var exec = require("child_process").exec;
function start(response) {
console.log("Request handler 'start' was called.");
exec("ls -lah", function (error, stdout, stderr) {
response.writeHead(200, {"Content-Type": "text/plain"});
response.write(stdout);
response.end();
});
}
function upload(response) {
console.log("Request handler 'upload' was called.");
response.writeHead(200, {"Content-Type": "text/plain"});
response.write("Hello Upload");
response.end();
}
exports.start = start;
exports.upload = upload;
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:
var exec = require("child_process").exec;
function start(response) {
console.log("Request handler 'start' was called.");
exec("find /",
{ timeout: 10000, maxBuffer: 20000*1024 },
function (error, stdout, stderr) {
response.writeHead(200, {"Content-Type": "text/plain"});
response.write(stdout);
response.end();
});
}
function upload(response) {
console.log("Request handler 'upload' was called.");
response.writeHead(200, {"Content-Type": "text/plain"});
response.write("Hello Upload");
response.end();
}
exports.start = start;
exports.upload = upload;
This will make HTTP requests to http://localhost:8888/start
take at most 10 seconds, but requests to http://localhost:8888/upload
will be answered immediately, even if /start is still
computing.
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
award-winning 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.
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:
function start(response) {
console.log("Request handler 'start' was called.");
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();
}
function upload(response) {
console.log("Request handler 'upload' was called.");
response.writeHead(200, {"Content-Type": "text/plain"});
response.write("Hello Upload");
response.end();
}
exports.start = start;
exports.upload = upload;
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/start
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
non-blocking 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
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.
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 http = require("http");
var url = require("url");
function start(route, handle) {
function onRequest(request, response) {
var postData = "";
var pathname = url.parse(request.url).pathname;
console.log("Request for " + pathname + " received.");
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);
});
}
http.createServer(onRequest).listen(8888);
console.log("Server has started.");
}
exports.start = start;
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:
function route(handle, pathname, response, postData) {
console.log("About to route a request for " + pathname);
if (typeof handle[pathname] === 'function') {
handle[pathname](response, postData);
} else {
console.log("No request handler found for " + pathname);
response.writeHead(404, {"Content-Type": "text/plain"});
response.write("404 Not found");
response.end();
}
}
exports.route = route;
And in requestHandlers.js, we include the data in our
response of the upload request handler:
function start(response, postData) {
console.log("Request handler 'start' was called.");
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();
}
function upload(response, postData) {
console.log("Request handler 'upload' was called.");
response.writeHead(200, {"Content-Type": "text/plain"});
response.write("You've sent: " + postData);
response.end();
}
exports.start = start;
exports.upload = upload;
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:
var querystring = require("querystring");
function start(response, postData) {
console.log("Request handler 'start' was called.");
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();
}
function upload(response, postData) {
console.log("Request handler 'upload' was called.");
response.writeHead(200, {"Content-Type": "text/plain"});
response.write("You've sent the text: "+
querystring.parse(postData).text);
response.end();
}
exports.start = start;
exports.upload = upload;
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 Geisendörfer. 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 corresponding 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
npm install formidable
on our command line. If the following output ends with
npm info build Success: formidable@1.0.9
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:
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(err, fields, files) {
res.writeHead(200, {'content-type': 'text/plain'});
res.write('received upload:\n\n');
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 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 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");
function start(response, postData) {
console.log("Request handler 'start' was called.");
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();
}
function upload(response, postData) {
console.log("Request handler 'upload' was called.");
response.writeHead(200, {"Content-Type": "text/plain"});
response.write("You've sent the text: "+
querystring.parse(postData).text);
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.writeHead(500, {"Content-Type": "text/plain"});
response.write(error + "\n");
response.end();
} else {
response.writeHead(200, {"Content-Type": "image/png"});
response.write(file, "binary");
response.end();
}
});
}
exports.start = start;
exports.upload = upload;
exports.show = show;
We also need to map this new request handler to the URL
/show in file index.js:
var server = require("./server");
var router = require("./router");
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/show
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/form-data 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:
var querystring = require("querystring"),
fs = require("fs");
function start(response, postData) {
console.log("Request handler 'start' was called.");
var body = '<html>'+
'<head>'+
'<meta http-equiv="Content-Type" '+
'content="text/html; charset=UTF-8" />'+
'</head>'+
'<body>'+
'<form action="/upload" enctype="multipart/form-data" '+
'method="post">'+
'<input type="file" name="upload">'+
'<input type="submit" value="Upload file" />'+
'</form>'+
'</body>'+
'</html>';
response.writeHead(200, {"Content-Type": "text/html"});
response.write(body);
response.end();
}
function upload(response, postData) {
console.log("Request handler 'upload' was called.");
response.writeHead(200, {"Content-Type": "text/plain"});
response.write("You've sent the text: "+
querystring.parse(postData).text);
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.writeHead(500, {"Content-Type": "text/plain"});
response.write(error + "\n");
response.end();
} else {
response.writeHead(200, {"Content-Type": "image/png"});
response.write(file, "binary");
response.end();
}
});
}
exports.start = start;
exports.upload = upload;
exports.show = show;
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.
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:
var http = require("http");
var url = require("url");
function start(route, handle) {
function onRequest(request, response) {
var pathname = url.parse(request.url).pathname;
console.log("Request for " + pathname + " received.");
route(handle, pathname, response, request);
}
http.createServer(onRequest).listen(8888);
console.log("Server has started.");
}
exports.start = start;
Next comes router.js - we don't need to pass postData
on anymore, and instead pass request:
function route(handle, pathname, response, request) {
console.log("About to route a request for " + pathname);
if (typeof handle[pathname] === 'function') {
handle[pathname](response, request);
} else {
console.log("No request handler found for " + pathname);
response.writeHead(404, {"Content-Type": "text/html"});
response.write("404 Not found");
response.end();
}
}
exports.route = route;
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:
var querystring = require("querystring"),
fs = require("fs"),
formidable = require("formidable");
function start(response) {
console.log("Request handler 'start' was called.");
var body = '<html>'+
'<head>'+
'<meta http-equiv="Content-Type" '+
'content="text/html; charset=UTF-8" />'+
'</head>'+
'<body>'+
'<form action="/upload" enctype="multipart/form-data" '+
'method="post">'+
'<input type="file" name="upload" multiple="multiple">'+
'<input type="submit" value="Upload file" />'+
'</form>'+
'</body>'+
'</html>';
response.writeHead(200, {"Content-Type": "text/html"});
response.write(body);
response.end();
}
function upload(response, request) {
console.log("Request handler 'upload' was called.");
var form = new formidable.IncomingForm();
console.log("about to parse");
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(err) {
if (err) {
fs.unlink("/tmp/test.png");
fs.rename(files.upload.path, "/tmp/test.png");
}
});
response.writeHead(200, {"Content-Type": "text/html"});
response.write("received image:<br/>");
response.write("<img src='/show' />");
response.end();
});
}
function show(response) {
console.log("Request handler 'show' was called.");
fs.readFile("/tmp/test.png", "binary", function(error, file) {
if(error) {
response.writeHead(500, {"Content-Type": "text/plain"});
response.write(error + "\n");
response.end();
} else {
response.writeHead(200, {"Content-Type": "image/png"});
response.write(file, "binary");
response.end();
}
});
}
exports.start = start;
exports.upload = upload;
exports.show = show;
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 wiki
and the NodeCloud directory
are probably the best starting points for more information.
Oh, one more thing: If you would like to dig a bit further into JavaScript
itself, you might consider reading
Object-orientation and inheritance in JavaScript: a comprehensive explanation
over at my blog.