12_browser.txt

:chap_num: 12
:prev_link: 11_language
:next_link: 13_dom

= JavaScript and the Browser =

[chapterquote="true"]
[quote,Douglas Crockford,The JavaScript Programming Language (video lecture)]
____
The browser is a really hostile programming environment.
____

The next part of this book will talk about web browsers. Without web
browsers, there would be no JavaScript. And even if there were, no
one would ever have paid any attention to it.

Web technology has, from the very start, been decentralized, not just
technically, but also in the way it has evolved. Various browser vendors
have added new functionality in ad-hoc and sometimes poorly thought out
ways, which then sometimes ended up being adopted by others, and
finally set down as a standard.

This is both a blessing and a curse. On the one hand, it is empowering
to not have a central party control a system, but have it be improved
by various parties working in loose collaboration (or, occasionally,
open hostility). On the other hand, the haphazard way in which the Web
was developed means that the resulting system is not exactly a shining
example of internal consistency. In fact, some parts of it are
downright messy and confusing.

== Networks and the Internet ==

Computer networks have been around since the 1950s. If you put cables
between two or more computers, and allow them to send data back and
forth through these cables, you can do all kinds of wonderful
things.

If connecting two machines in the same building allows us to do
wonderful things, connecting machines all over the planet should be
even better. The technology to start implementing this vision was
developed in the 1980s, and the resulting network is called the
_Internet_. It has lived up to its promise.

A computer can use this network to spew bits at another computer. For
any effective communication to arise out of this bit-spewing, the
computers at both ends must know what the bits are supposed to
represent. The meaning of any given sequence of bits depends entirely
on the kind of thing that it is trying to express, and the encoding
mechanism used.

A _network protocol_ describes a style of communication over
a network. There are protocols for sending email, fetching email,
sharing files, or even for controlling computers that happen to be
infected by malicious software.

For example, a simple chat protocol might consist of one computer
sending the bits that represent the text “CHAT?” to another
machine, and the other responding with “OK!” to confirm that it
understands the protocol. They can then proceed to send each other
strings of text, read the text sent by the other from the network,
and display whatever they receive on their screens.

Most protocols are built on top of other protocols. Our example chat
protocol treats the network as a stream-like device into which you
can put bits, and have them arrive at the correct destination in the
correct order. Ensuring those things is already a rather difficult
technical problem.

TCP (_Transmission Control Protocol_) is a protocol that solves this
problem. All internet-connected devices “speak” it, and most
communication on the Internet is built on top of it.

A TCP connection works as follows: one computer must be waiting, or
_listening_, for other computers to start talking to it. To be able to
listen for different kinds of communication at the same time on a
single machine, each listener has a number (called a _port_) associated
with it. Most protocols specify which port should be used by default.
For example, when we want to send an email using the SMTP protocol,
the machine through which we send it is expected to be listening on
port 25.

Another computer can then establish a connection by connecting to the target
machine using the correct port number. If the target machine can be
reached, and is listening on that port, the connection is successfully
created. The listening computer is called the _server_, and the
connecting computer the _client_.

Such a connection acts as a two-way pipe through which bits can
flow—the machines on both ends can put data into it. Once the bits
are successfully transmitted, they can be read out again by the machine on the
other side. This is a very convenient model. You could say that TCP
provides an abstraction of the network.

[[web]]
== The Web ==

The _World Wide Web_ (not to be confused with the Internet as a whole)
is a set of protocols and formats that allow us to visit web pages in a
browser. The “Web” part in the name refers to the fact that such pages
can easily link to each other, thus connecting into a huge mesh that
users can move through.

To add content to the Web, all you need to do is connect a machine
to the Internet, and have it listen on port 80, using HTTP (the
_Hyper-Text Transfer Protocol_). This protocol allows other computers
to request documents over the network.

Each document on the Web is named by a URL (_Universal Resource Locator_), which
looks something like this:

----
  http://eloquentjavascript.net/12_browser.html
 |      |                      |               |
 protocol       server               path
----

The first part tells us that this URL uses the HTTP protocol (as
opposed to, for example, encrypted HTTP, which would be
_https://_). Then comes the part that identifies which server we are
requesting the document from. Last is a path string that
identifies the specific document (or _resource_) we are interested in.

Each machine connected to the Internet gets a unique _IP
address_, which looks something like `37.187.37.82`. You can use
these directly as the server part of a URL. But lists of more or less
random numbers are hard to remember and awkward to type, so
you can instead register a _hostname_ to point towards a
specific machine or set of machines. I registered
_eloquentjavascript.net_ to point at the IP address of a machine I
control, and can thus use that hostname to serve web pages.

If you type the URL above into your browser's address bar, it will try
to retrieve and display the document at that URL. First, your browser has to find out what address
_eloquentjavascript.net_ refers to. Then, using the HTTP protocol, it
makes a connection to the server at that address, and asks for the
file _/12_browser.html_.

We will take a closer look at the HTTP protocol in link:17_http.html#http[Chapter 17].

== HTML ==

HTML, which stands for _Hyper-Text Markup Language_, is the document
format used for web pages. An HTML document contains text, as well as
_tags_ that gives structure to the text, describing things
like links, paragraphs, and headings.

A simple HTML document looks like this:

[source,text/html]
----
<!doctype html>
<html>
  <head>
    <title>My home page</title>
  </head>
  <body>
    <h1>My home page</h1>
    <p>Hello, I am Marijn and this is my home page.</p>
    <p>I also wrote a book! Read it
      <a href="http://eloquentjavascript.net">here</a>.</p>
  </body>
</html>
----

ifdef::tex_target[]

This is what such a document would look like in the browser:

image::img/home-page.png[alt="My home page",width="6.3cm"]

endif::tex_target[]

The tags, wrapped in angular brackets (“++<++” and “++>++”), provide
information about the structure of the document. The other text is
just plain text.

The document starts with `<!doctype html>`, which tells the browser to
interpret it as _modern_ HTML, as opposed to various dialects that
were in use in the past.

HTML documents have a head and a body. The head contains information
_about_ the document, and the body contains the document itself. In
this case, we first declared that the title of this document is “My
home page”, and then gave a document containing a heading (`<h1>`,
meaning “heading 1”—++<h2>++ to `<h6>` produce more minor headings)
and two paragraphs (`<p>`).

Tags come in several forms. An element, such as the
body, a paragraph, or a link, is started by an _opening tag_ like
`<p>`, and ended by a _closing tag_ like `</p>`. Some
opening tags, such as the one for the link (`<a>`) contain extra
information, in the form of `name="value"` pairs. These are called
_attributes_. In this case, the destination of the link is indicated
with `href="http://eloquentjavascript.net"`, where `href` stands for
“hypertext reference”.

Some kinds of tags do not enclose anything, and thus do not
need to be closed. An example of this would be `<img
src="http://example.com/image.jpg">`, which will display the image
found at the given source URL.

To be able to include angular brackets in the text of a document, even
though they have a special meaning in HTML, yet another form of
special notation has to be introduced. A plain opening angular bracket
is written as `&lt;` (“less than”), and a closing bracket as `&gt;`
(“greater than”). In HTML, an ampersand (“&”) character followed by a
word and a semicolon (“;”) is called an _entity_, and will be replaced
by the character it encodes.

This is analogous to the way backslashes are used in JavaScript strings.
Since this mechanism gives ampersand characters a special meaning too,
those need to be escaped as `&amp;`. Inside of an attribute, which is wrapped
in double quotes, `&quot;` can be used to insert an actual quote character.

HTML is parsed in a remarkably error-tolerant way. When tags that
should be there are missing, the browser reconstructs them. The way in
which this is done has been standardized, and you can rely on all modern
browsers to do it in the same way.

The following document will be treated just like the one above:

[source,text/html]
----
<!doctype html>

<title>My home page</title>

<h1>My home page</h1>
<p>Hello, I am Marijn and this is my home page.
<p>I also wrote a book! Read it
  <a href=http://eloquentjavascript.net>here</a>.
----

The `<html>`, `<head>` and `<body>` tags are gone completely. The
browser knows that `<title>` belongs in a head, and `<h1>` in a body.
Furthermore, I am no longer explicitly closing the paragraphs, since
opening a new paragraph or ending the document will
close them implicitly. The quotes around the link target are also
gone.

This book will usually omit the `<html>`, `<head>`, and `<body>` tags
from examples, to keep them short and free of clutter. But I _will_
close tags and include quotes around attributes.

I will also usually omit the doctype. This is not to be taken as an
encouragement to omit doctype declarations. Browsers will often do
ridiculous things when you forget them. You should consider doctypes
implicitly present in examples, even when they are not actually
shown in the text.

[[script_tag]]
== HTML and JavaScript ==

In the context of this book, the most important HTML tag is
`<script>`. This tag allows us to include a piece of JavaScript in a
document.

[source,text/html]
----
<h1>Testing alert</h1>
<script>alert("hello!");</script>
----

Such a script will run as soon as its `<script>` tag is encountered
as the browser reads the HTML. The page above will pop up an `alert`
dialog when opened.

Including large programs directly in HTML documents is
often impractical. The `<script>` tag can be given an `src`
attribute in order to fetch a script file (a text file containing
a JavaScript program) from a URL.

[source,text/html]
----
<h1>Testing alert</h1>
<script src="code/hello.js"></script>
----

The _code/hello.js_ file included here contains the same simple program,
`alert("hello!")`. When an HTML page references other URLs as part of
itself, for example an image file or a script, web browsers will
retrieve them immediately, and include them in the page.

A script tag must always be closed with `</script>`, even if it refers
to a script file and doesn't contain any code. If you forget this, the
rest of the page will be interpreted as part of the script.

Some attributes can also contain a JavaScript program. The `<button>`
tag below (which shows up as a button) has an `onclick`
attribute, whose content will be run whenever the button is pressed.

[source,text/html]
----
<button onclick="alert('Boom!')">DO NOT PRESS</button>
----

Note that I had to use single quotes for the string in the `onclick`
attribute, because double quotes are already used to quote the whole
attribute. I could also have used `&quot;`, but that'd make the
program harder to read.

== In the sandbox ==

Running programs downloaded from the Internet is potentially dangerous.
You do not know much about the people behind most sites you visit, and
they do not necessarily mean well. Running programs by people who do
not mean well is how you get your computer infected by viruses, your
data stolen, and your accounts hacked.

Yet the attraction of the Web is that you can surf it without necessarily
trusting all the pages you visit. This is why browsers severely limit
the things a JavaScript program may do: it can't look at
the files on your computer, or modify anything not related to the
web page it was embedded in.

Isolating a programming environment in this way is called _sandboxing_,
the idea being that the program is harmlessly playing in a sandbox. But
you should imagine this particular kind of sandbox as having a cage of
thick steel bars over it, which makes it somewhat different
from your typical playground sandbox.

The hard part of sandboxing is allowing the programs enough room to be
useful, yet at the same time restricting them from doing anything
dangerous. Lots of useful functionality,
such as communicating with other servers, or reading the
content of the copy-paste clipboard, can also be used to do
problematic, privacy-invading things.

Every now and then, someone comes up with a new way to circumvent the
limitations of a browser and do something harmful, ranging from
leaking minor private information to taking over the whole machine
that the browser runs on. The browser developers respond by fixing the loophole, and
all is well again. That is, until the next problem is discovered—and hopefully
publicized, rather than secretly exploited by some government or mafia.

== Compatibility and the browser wars ==

In the very early stages of the Web, a browser called Mosaic dominated
the market. After a few years, the balance had shifted to Netscape,
which was then, in turn, largely supplanted by Microsoft's Internet
Explorer. At any point where a single browser was dominant, that
browser's vendor would feel entitled to unilaterally invent new
features for the Web. Since most users used the same browser, web
sites would simply start using those features—never mind the other
browsers.

This was the dark age of compatibility, often called the “browser
wars”. Web developers were left with not one unified Web, but two or three
incompatible platforms. To make things worse, the browsers in use
around 2003 were all full of bugs, and of course the bugs were
different for each browser. Life was hard for people writing web
pages.

Mozilla Firefox, a not-for-profit offshoot of Netscape, challenged
Internet Explorer's hegemony in the late 2000s. Because Microsoft
was not particularly interested in staying competitive at the time,
Firefox took quite a chunk of market share away from them. Around the same time,
Google introduced its Chrome browser, and Apple's Safari browser
gained popularity, leading to a situation where there were four major
players, rather than one.

The new players had a more serious attitude towards standards
and better engineering practices, leading to less incompatibility
and fewer bugs. Microsoft, seeing its market share crumble, came
around and adopted these attitudes. If you are starting to learn web
development today, consider yourself lucky. The latest versions of the
major browsers behave quite uniformly, and have relatively few bugs.

Which is not to say that the situation is perfect just yet. Some of
the people using the Web are, for reasons of inertia or corporate
policy, stuck with very old browsers. Until those browsers die out entirely,
writing websites that work for them will require a lot of arcane
knowledge about their shortcomings and quirks. This book is not about
those quirks. Rather, it aims to present the modern, sane style of
web programming.