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17_http.txt
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:chap_num: 17
:prev_link: 16_canvas
:next_link: 18_forms
:load_files: ["code/chapter/17_http.js", "js/promise.js"]
= HTTP =
[chapterquote="true"]
[quote,Tim Berners-Lee,The World Wide Web: A very short personal history]
____
The dream behind the Web is of a common information space in which we
communicate by sharing information. Its universality is essential: the
fact that a hypertext link can point to anything, be it personal,
local or global, be it draft or highly polished.
____
(((Berners-Lee+++,+++ Tim)))(((World Wide Web)))(((HTTP)))The
_Hyper-Text Transfer Protocol_, already mentioned in
link:12_browser.html#web[Chapter 12], is the mechanism through which
data is requested and provided on the ((World Wide Web)). This chapter
describes the ((protocol)) in detail, and explains the way ((browser))
JavaScript has access to it.
== The protocol ==
(((IP address)))If you type _eloquentjavascript.net/17_http.html_ into
your browser's ((address bar)), the ((browser)) first looks up the
((address)) of the server associated with _eloquentjavascript.net_,
and tries to open a ((TCP)) ((connection)) to it on ((port)) 80, the
default port for ((HTTP)) traffic. If the ((server)) exists and
accepts the connection, the browser sends something like this:
[source,http]
----
GET /17_http.html HTTP/1.1
Host: eloquentjavascript.net
User-Agent: Your browser's name
----
After which the server responds, through that same connection:
[source,http]
----
HTTP/1.1 200 OK
Content-Length: 3122
Content-Type: text/html
Last-Modified: Wed, 09 Apr 2014 10:48:09 GMT
<!doctype html>
... the rest of the document
----
The browser then takes the part of the ((response)) below the blank
line, and displays it as an ((HTML)) document.
(((HTTP)))The information sent by the client is called the
_((request))_. It starts with this line:
[source,http]
----
GET /17_http.html HTTP/1.1
----
(((DELETE method)))(((PUT method)))(((GET method)))The first word is
the _((method))_ of the ((request)). `GET` means that we want to _get_
the specified resource. Other common methods are `DELETE` to delete a
resource, `PUT` to replace it, and `POST` to send information to it.
Note that the ((server)) is not obliged to carry out every request it
gets. If you walk up to a random website and tell it to `DELETE` its
main page, it'll probably refuse.
(((path,URL)))(((Twitter)))The part after the ((method)) name is the path of the
((resource)) the request applies to. In the simplest case, a resource
is simply a ((file)) on the ((server)). But the protocol doesn't
require it to be: it may be anything that can be transferred _as if_
it is a file. Many servers generate the responses they produce on the
fly. For example, if you open
http://twitter.com/marijnjh[_twitter.com/marijnjh_], the server looks
in its database for a user named “marijnjh”, and if it finds one, it
will generate a profile page for that user.
After the resource path, the first line of the request mentions
`HTTP/1.1`, to indicate the ((version)) of the ((HTTP)) ((protocol))
it is using.
(((status code)))The server's ((response)) will start with a version
as well, followed by the status of the response, first as a
three-digit code, and then as a human-readable string.
[source,http]
----
HTTP/1.1 200 OK
----
(((200 (HTTP status code))))(((error response)))(((404 (HTTP status
code))))Codes starting with a 2 indicate that the request succeeded.
Codes starting with a 4 mean there was something wrong with the
((request)). 404 is probably the most famous HTTP status code—it means
that the resource that was requested could not be found. Codes that
start with 5 mean an error happened on the the ((server)), and the
request is not to blame.
[[headers]]
(((HTTP)))The first line of a request or response may be followed by
any number of _((header))s_. These are lines in the form “name: value”
that specify extra information about the request or response. These
headers were part of the example ((response)):
----
Content-Length: 65585
Content-Type: text/html
Last-Modified: Wed, 09 Apr 2014 10:48:09 GMT
----
(((Content-Length header)))(((Content-Type header)))(((Last-Modified
header)))This tells us the size and type of the response document. In
this case, an HTML document of 65,585 bytes. It also tells us when
that document was last modified.
(((Host header)))(((domain)))Which ((header))s to include in a
((request)) or a ((response)) is mostly up to the client or server
sending it, though a few are required. For example, the `Host` header
in a request, which specifies the host name, should be included,
because a ((server)) might be serving multiple host names on a single
((IP address)), and without that header it won't know which one the
client is trying to talk to.
(((GET method)))(((DELETE method)))(((PUT method)))(((POST
method)))(((body (HTTP))))After the headers, both requests and
responses may include a blank line followed by a _body_, which
contains the data being sent. `GET` or `DELETE` request don't send
along any data, but `PUT` or `POST` requests are expected to.
Similarly, some response types, such as error responses, do not
require a body.
== Browsers and HTTP ==
(((HTTP)))As we saw in the example, a ((browser)) will make a request
when we enter a ((URL)) in its ((address bar)). When the resulting
HTML page references other files, such as ((image))s and JavaScript
((file))s, those are also fetched.
(((parallelism)))A moderately complicated ((website)) can easily
include anywhere from ten to two hundred ((resource))s. To be able to
fetch those quickly, browsers will make several requests
simultaneously, rather than waiting for the responses one at a time.
(((GET method)))Fetching such documents is always done using `GET`
((request))s.
[[http_forms]]
(((POST method)))Generating `POST` requests in a ((browser)) is also a
common thing to do. HTML pages may include _((form))s_, which allow
the user to fill out information and send it to the server. This is an
example of a form:
[source,text/html]
----
<form method="GET" action="example/message.html">
<p>Name: <input type="text" name="name"></p>
<p>Message:<br><textarea name="message"></textarea></p>
<p><button type="submit">Send</button></p>
</form>
----
(((form)))(((method attribute)))(((GET method)))That will show two
((field))s, a small one asking for a name, and a larger one to write a
message in. When you press the “Send” ((button)), the information in
the fields will be encoded into a _((query string))_. When the
`<form>` element's `method` attribute is `GET` (or is omitted), that
query string is tacked onto the `action` URL, and the browser makes a
`GET` request to that URL.
[source,text/html]
----
GET /example/message.html?name=Jean&message=Yes%3F HTTP/1.1
----
(((ampersand character)))The start of a ((query string)) is indicated
by a ((question mark)). After that follow pairs of names and values,
corresponding to the `name` attribute on the form field elements, and
the content of those elements. The ampersand (“&”) is used to separate
the pairs.
(((escaping,in URLs)))(((hexadecimal number)))(((percent
sign)))(((URL encoding)))(((encodeURIComponent
function)))(((decodeURIComponent function)))The actual message encoded
in the URL above is “Yes?”. Some characters in query strings must be
escaped. The question mark is one of those, and is represented as
`%3F`. There seems to be an unwritten rule that every format needs its
own different way of escaping characters. This one, called “URL
encoding”, uses a percent sign followed by two hexadecimal digits
which encode the character code. In this case 3F, which is 63 in
decimal notation, is the code of a question mark character. JavaScript
provides the `encodeURIComponent` and `decodeURIComponent` functions
to en- and decode this format.
[source,javascript]
----
console.log(encodeURIComponent("Hello & goodbye"));
// → Hello%20%26%20goodbye
console.log(decodeURIComponent("Hello%20%26%20goodbye"));
// → Hello & goodbye
----
(((body (HTTP))))(((POST method)))If we change the `method` attribute
of the form above to `POST`, the ((HTTP)) request made to submit the
((form)) will use the `POST` method, and put the ((query string)) in
body of the request, rather than adding it to the URL.
[source,http]
----
POST /example/message.html HTTP/1.1
Content-length: 24
Content-type: application/x-www-form-urlencoded
name=Jean&message=Yes%3F
----
The link:18_forms.html#forms[next chapter] will come back to forms,
and talk about the way we can script them with JavaScript.
[[xmlhttprequest]]
== XMLHttpRequest ==
(((capitalization)))(((XMLHttpRequest)))The ((interface)) through
which browser JavaScript can make HTTP requests is called
`XMLHttpRequest` (note the inconsistent capitalization). It was
designed by ((Microsoft)), for their ((Internet Explorer))
((browser)), in the late 1990s. During this time, in the world of
((business software)) (a world which Microsoft has always been at home
in) the ((XML)) file format was _very_ popular. So popular that the
word “XML” was tacked onto the front of the name of an interface for
((HTTP)), which is in no way tied to XML.
(((modularity)))(((interface,design)))The name isn't completely
nonsensical. The interface allows you to parse response documents as
XML if you want to. Confusing two distinct concepts (making a request
and ((parsing)) the response) into a single thing is terrible design,
of course, but so it goes.
When the `XMLHttpRequest` interface was added to Internet Explorer, it
allowed people to do things with JavaScript that had been very hard
before. For example, websites started showing lists of suggestions
when the user was typing something into a text field. The script would
send the text typed to the server over ((HTTP)), and the ((server)),
which had some ((database)) of things that the user might mean, would
match those against the partial input, and send back possible
((completion))s, which were then shown to the user. This was
considered very spectacular—people were used to the fact that every
interaction with a website required a full page reload.
(((compatibility)))(((Firefox)))(((XMLHttpRequest)))The other
significant browser at that time, ((Mozilla)) (later Firefox) did not
want to be left behind. To allow people to do similarly neat things in
_their_ browser, they copied the interface, including the bogus name.
The next generation of ((browser))s followed this example, and today
`XMLHttpRequest` is a de facto standard ((interface)).
== Sending a request ==
(((open method)))(((send method)))(((XMLHttpRequest)))To make a simple
((request)), we create a request object with the `XMLHttpRequest`
constructor, and call its `open` and `send` methods.
// test: trim
[source,javascript]
----
var req = new XMLHttpRequest();
req.open("GET", "example/data.txt", false);
req.send(null);
console.log(req.responseText);
// → This is the content of data.txt
----
(((path,URL)))(((open method)))(((relative URL)))(((slash character)))The `open`
method configures the request. In this case, we choose to make a `GET`
request for the _example/data.txt_ file. ((URL))s that don't start
with a protocol (like _http://_) are called relative, which means that
they are interpreted relative to the current document. When they start
with a slash (“/”), they replace the current path (the part after the
server name). When they do not, the part of the current path up to
and including its last slash character is put in front of the relative
URL.
(((send method)))(((GET method)))(((body (HTTP))))(((responseText
property)))After opening the request, we can send it with the `send`
method. The argument to send is the request body. For `GET` requests,
we can pass null. If the third argument to `open` was `false`, `send`
will only return after the response to our request was received. We
can read the request object's `responseText` property to get the
response body.
(((status property)))(((statusText
property)))(((header)))(((getResponseHeader method)))The other
information included in the response can also be extracted from this
object. The ((status code)) is accessible through the `status`
property, and the human-readable status text through `statusText`.
Headers can be read with `getResponseHeader`.
// test: no
[source,javascript]
----
var req = new XMLHttpRequest();
req.open("GET", "example/data.txt", false);
req.send(null);
console.log(req.status, req.statusText);
// → 200 OK
console.log(req.getResponseHeader("content-type"));
// → text/plain
----
(((case-sensitivity)))(((capitalization)))Headers names are
case-insensitive. They are usually written with a capital letter at
the start of each word, such as “Content-Type”, but “content-type” or
“cOnTeNt-TyPe” refer to the same header.
(((Host header)))(((setRequestHeader method)))The browser will
automatically add some request ((header))s, such as “Host” and those
needed for the server to figure out the size of the body. But you can
add your own headers with the `setRequestHeader` method. This is only
needed for advanced uses, and requires the cooperation of the
((server)) you are talking to—a server is free to ignore headers it
does not know how to handle.
== Asynchronous Requests ==
(((XMLHttpRequest)))(((event handling)))(((blocking)))(((synchronous
I/O)))(((responseText property)))(((send method)))In the examples we
saw, the request has finished when the call to `send` returns. This is
convenient, because it means properties like `responseText` are
available immediately. But it does mean that our program is suspended
as long as the ((browser)) and server are communicating. When the
((connection)) is bad, the server is slow, or the file is big, that
might take quite a while. Worse, because no event handlers can fire
while our program is suspended, the whole document will become
unresponsive.
(((XMLHttpRequest)))(((open method)))(((asynchronous I/O)))If we pass
`true` as the third argument to `open`, the request is _asynchronous_.
That means that when we call `send`, the only thing that happens right
away is that the request gets scheduled to be sent. Our program can
continue, and the browser will take care of the sending and receiving
of data in the background.
But as long as the request is running, we won't be able to access the
response. We need a mechanism that will notify us when the data is
available.
(((event handling)))(((load event)))For this, we must listen for the
`"load"` event on the request object.
[source,javascript]
----
var req = new XMLHttpRequest();
req.open("GET", "example/data.txt", true);
req.addEventListener("load", function() {
console.log("Done:", req.status);
});
req.send(null);
----
(((asynchronous programming)))(((callback function)))Just like the use
of `requestAnimationFrame` in link:15_game.html#game[Chapter 15], this
forces us to use an asynchronous style of programming, wrapping the
things that have to be done after the request in a function, and
arranging for that to be called at the appropriate time. We will come
back to this link:17_http.html#promises[later].
== Fetching XML Data ==
(((documentElement property)))(((responseXML property)))When the
resource retrieved by an `XMLHttpRequest` object is an ((XML))
document, the object's `responseXML` property will hold a parsed
representation of this document. This representation works much like
the ((DOM)) discussed in link:13_dom.html#dom[Chapter 13], except that
it doesn't have HTML-specific functionality like the `style` property.
The object that `responseXML` holds corresponds to the `document`
object. Its `documentElement` property refers to the outer tag of the
XML document. In the document below (_example/fruit.xml_), that would
would be the `<fruits>` tag.
[source,application/xml]
----
<fruits>
<fruit name="banana" color="yellow"/>
<fruit name="lemon" color="yellow"/>
<fruit name="cherry" color="red"/>
</fruits>
----
We can retrieve it like this:
// test: no
[source,javascript]
----
var req = new XMLHttpRequest();
req.open("GET", "example/fruit.xml", false);
req.send(null);
console.log(req.responseXML.querySelectorAll("fruit").length);
// → 3
----
(((data format)))XML documents can be used to exchange structured
information with the server. Their form—tags nested inside other
tags—lends itself well to storing most types of data, or at least
better than flat text files. The DOM interface is rather clumsy for
extracting information, though, and ((XML)) documents tend to be
verbose. It is often a better idea to communicate using ((JSON)) data.
[source,javascript]
----
var req = new XMLHttpRequest();
req.open("GET", "example/fruit.json", false);
req.send(null);
console.log(JSON.parse(req.responseText));
// → {banana: "yellow", lemon: "yellow", cherry: "red"}
----
[[http_sandbox]]
== HTTP sandboxing ==
(((sandbox)))Making ((HTTP)) requests in web page scripts once
again raises concerns about ((security)). The person who controls the
script might not have the same interests as the person on whose
computer it is running. More specifically, if I visit _themafia.org_,
I do not want its scripts to be able to make a request to
_mybank.com_, using identifying information from my ((browser)), with
instructions to transfer all my money to some random ((mafia))
account.
It is not too hard for ((website))s to protect themselves against such
((attack))s, but it requires effort, and many websites fail to do it.
For this reason, browsers protect us by disallowing scripts to make
HTTP requests to other _((domain))s_ (names like _themafia.org_ and
_mybank.com_).
(((Access-Control-Allow-Origin header)))(((cross-domain request)))This
can be an annoying problem when building systems that want to access
several domains for legitimate reasons. It is possible for ((server))s
to include a ((header)) like this in their ((response)) to explicitly
indicate to browsers that it is okay for the request to come from a
different domain:
----
Access-Control-Allow-Origin: *
----
== Abstracting requests ==
(((HTTP)))(((XMLHttpRequest)))(((backgroundReadFile function)))In
link:10_modules.html#amd[Chapter 10], in our implementation of the AMD
module system, we used a hypothetical function called
`backgroundReadFile`. It took a file name and a function, and called
that function with the contents of the file when it had finished
fetching it. Here's a simple implementation of that function:
// include_code
[source,javascript]
----
function backgroundReadFile(url, callback) {
var req = new XMLHttpRequest();
req.open("GET", url, true);
req.addEventListener("load", function() {
if (req.status < 400)
callback(req.responseText);
});
req.send(null);
}
----
(((XMLHttpRequest)))This simple ((abstraction)) makes it easier to use
`XMLHttpRequest` for simple `GET` requests. If you are writing a
program that has to make HTTP requests, it is a good idea to use a
helper function, so that you don't end up repeating the ugly
`XMLHttpRequest` pattern all through your code.
(((function,as value)))(((callback function)))The function argument's
name, `callback`, is a term that is often used to describe functions
like this. A callback function is given to other code to provide that
code with a way to “call us back” later.
(((library)))It is not hard to write your own, tailored to what your
application is doing. The above one only does `GET` requests, and
doesn't give us control over the headers or the request body. You
could write another variant for `POST` requests, or a more generic one
that supports various kinds of requests. Many JavaScript libraries
also provide with wrappers for `XMLHttpRequest`.
(((user experience)))(((error response)))The main problem with the
wrapper above is its handling of ((failure)). When the request returns
a ((status code)) that indicates an error (400 and up), it does
nothing. This might be okay, in some circumstances, but imagine we put
a “loading” indicator on the page to indicate that we are fetching
information. If the request fails, because the server crashed or the
((connection)) is briefly interrupted, the page will just sit there,
misleadingly looking like it is doing something. The user will wait
for a while, get impatient, and consider the site uselessly flaky.
We should also have an option to be notified when the request fails,
so that we can take appropriate action—for example, remove the
“loading” message and inform the user that something went wrong.
(((exception handling)))(((callback function)))(((error
handling)))(((asynchronous programming)))(((try
keyword)))(((stack)))Error handling in asynchronous code is even more
tricky than error handling in synchronous code. Because we often need
to defer part of our work, putting it in a callback function, the
scope of a `try` block becomes meaningless. In the code below, the
exception will _not_ be caught, because the call to
`backgroundReadFile` returns immediately. Control then leaves the
`try` block, and the function it was given won't be called until
later.
// test: no
[source,javascript]
----
try {
backgroundReadFile("example/data.txt", function(text) {
if (text != "expected")
throw new Error("That was unexpected");
});
} catch (e) {
console.log("Hello from the catch block");
}
----
[[getURL]]
(((HTTP)))(((getURL function)))(((exception)))To handle failing
requests, we have to allow an additional function to be passed to our
wrapper, and call that when a request goes wrong. Or alternatively, we
can use the convention that if the request fails, an additional
argument describing the problem is passed to the regular callback
function. For example:
// include_code
[source,javascript]
----
function getURL(url, callback) {
var req = new XMLHttpRequest();
req.open("GET", url, true);
req.addEventListener("load", function() {
if (req.status < 400)
callback(req.responseText);
else
callback(null, new Error("Request failed: " +
req.statusText));
});
req.addEventListener("error", function() {
callback(null, new Error("Network error"));
});
req.send(null);
}
----
(((error event)))We have added a handler for the `"error"` event,
which will be signaled when the request fails entirely. We also call
the ((callback function)) with an error argument when the request
completes with a ((status code)) that indicates an error.
Code using `getURL` must then check whether an error was given, and if
it finds one, handle it.
[source,javascript]
----
getURL("data/nonsense.txt", function(content, error) {
if (error != null)
console.log("Failed to fetch nonsense.txt: " + error);
else
console.log("nonsense.txt: " + content);
});
----
(((uncaught exception)))(((exception handling)))(((try keyword)))This
does not help when it comes to exceptions. When chaining several
asynchronous actions together, an exception at any point of the chain
will still, unless you wrap each handling function in its own
`try`/`catch` block, land at the top level and abort your chain of
actions.
[[promises]]
== Promises ==
(((promise)))(((asynchronous programming)))(((callback
function)))(((readability)))(((uncaught exception)))For complicated
projects, writing asynchronous code in plain callback style is hard to
do correctly. It is easy to forget to check for an error, or to allow
an unexpected exception to cut the program short in a crude way.
Additionally, arranging for correct error handling when the error has
to flow through multiple callback functions and `catch` blocks is
tedious.
(((future)))(((ECMAScript 6)))There have been a lot of attempts to
solve this with extra abstractions. One of the more successful ones is
called _promises_. Promises wrap an asynchronous action in an object,
which can be passed around and told to do certain things when the
action finishes or fails. They are set to become a part of the next
version of the JavaScript language, but can already be used as a
library.
The ((interface)) for promises is somewhat non-obvious, but very
powerful. This chapter will only roughly describe it. A more thorough
treatment can be found at
https://www.promisejs.org/[_www.promisejs.org_].
(((Promise constructor)))To create a promise object, we call the
`Promise` constructor, giving it a function that will initialize the
asynchronous action. The constructor calls that function, passing it
two arguments, which are themselves functions. The first should be
called when the action finishes successfully, and the second when it
fails.
(((HTTP)))(((get function)))Here is once again our wrapper for `GET`
requests, this time returning a promise. We'll simply call it `get`,
this time.
// include_code
[source,javascript]
----
function get(url) {
return new Promise(function(succeed, fail) {
var req = new XMLHttpRequest();
req.open("GET", url, true);
req.addEventListener("load", function() {
if (req.status < 400)
succeed(req.responseText);
else
fail(new Error("Request failed: " + req.statusText));
});
req.addEventListener("error", function() {
fail(new Error("Network error"));
});
req.send(null);
});
}
----
Note that the ((interface)) to the function itself is now a lot
simpler. You give it a URL, and it returns a ((promise)). That promise
acts as a _handle_ to the request's outcome. It has a `then` method
that you can call with two functions, one to handle success, and one
to handle failure.
[source,javascript]
----
get("example/data.txt").then(function(text) {
console.log("data.txt: " + text);
}, function(error) {
console.log("Failed to fetch data.txt: " + error);
});
----
(((chaining)))So far, this is just a different way to express the same
thing we already expressed before. It is only when you need to chain
actions together that promises make a significant difference.
(((then method)))Calling `then` produces a new ((promise)), whose
result (the value passed to success handlers) depends on the return
value of the first function we passed to `then`. This function may
return another promise, to indicate that more asynchronous work is
being done. In this case, the promise returned by `then` itself will
wait for the promise returned by the handler function, succeeding or
failing with the same value when it is resolved. When the handler
function returns a non-promise value, the promise returned by `then`
immediately succeeds with that value as its result.
(((then method)))(((chaining)))This means you can use `then` to
transform the result of a promise. For example, this returns a promise
whose result is the content of the given URL, parsed as ((JSON)).
// include_code
[source,javascript]
----
function getJSON(url) {
return get(url).then(JSON.parse);
}
----
(((error handling)))That last call to `then` did not specify a failure
handler. This is allowed. The error will be passed on to the promise
returned by `then`, which is exactly what we want—`getJSON` does not
know what to do when something goes wrong, but its caller hopefully
does.
As an example that shows the use of ((promise))s, we will build a
program that fetches a number of JSON files from the server, and,
while it is doing that, shows the word “loading”. The JSON files
contain information about people, with links to files that represent
other people in properties like `father`, `mother`, or `spouse`.
(((error message)))(((JSON)))We want to get the name of the mother of
the spouse of _example/bert.json_. And if something goes wrong, we
want to remove the “loading” text and show an error message instead.
Here is how that might be done with ((promise))s:
[source,text/html]
----
<script>
function showMessage(msg) {
var elt = document.createElement("div");
elt.textContent = msg;
return document.body.appendChild(elt);
}
var loading = showMessage("Loading...");
getJSON("example/bert.json").then(function(bert) {
return getJSON(bert.spouse);
}).then(function(spouse) {
return getJSON(spouse.mother);
}).then(function(mother) {
showMessage("The name is " + mother.name);
}).catch(function(error) {
showMessage(String(error));
}).then(function() {
document.body.removeChild(loading);
});
</script>
----
(((error handling)))(((catch method)))(((then
method)))(((readability)))(((program size)))The resulting program is
relatively compact and readable. The `catch` method is similar to
`then`, except that it only expects a failure handler, and will pass
through the result unmodified in case of success. Much like with the
`catch` clause for the `try` statement, control will continue as
normal after the failure is caught, so that the final `then`, which
removes the loading message, is always executed, even if something
went wrong.
(((asynchronous programming)))(((domain-specific language)))You can
think of the promise interface as implementing its own language for
asynchronous ((control flow)). The extra method calls and function
expressions needed to achieve this make the code look somewhat
awkward, but not remotely as awkward as it would look if we took care
of all the error handing ourselves.
== Appreciating HTTP ==
(((client)))(((HTTP)))When building a system that requires
((communication)) between a JavaScript program running in the
((browser)) (client-side) and a program on a ((server)) (server-side),
there several different ways to model this communication.
(((network)))(((abstraction)))A commonly used model is that of
_((remote procedure call))s_. In this model, communication follows the
patterns of normal function calls, except that the function is
actually running on another machine. Calling it involves making a
request to the server that includes the function's name and arguments.
The response to that request contains the returned value.
When thinking in terms of remote procedure calls, HTTP is just a
vehicle for communication, and you will most likely write an
abstraction layer that hides it entirely.
(((media type)))(((document format)))Another approach is to build your
communication around the concept of ((resource))s and ((HTTP))
((method))s. Instead of a remote procedure called `addUser`, you use a
`PUT` request to `/users/larry`. Instead of encoding that user's
properties in function arguments, you define a document format (or use
an existing one) that represents the user, and use it as the body of
this `PUT` request. Fetching a resource is done by making `GET`
request to the resource's URL, for example `/user/larry`, which
returns the document representing that resource.
This second approach makes it easier to use some of the features that
HTTP provides, such as support for caching resources (keeping a copy
on the client side). It can also help the coherence of your interface,
since resources are easier to reason about than a jumble of functions.
== Security and HTTPS ==
(((man-in-the-middle)))(((security)))(((HTTPS)))Data traveling over
the internet tends to follow a long, dangerous road. In order to get
to its destination it must hop through anything from coffee shop wifi
((network))s to networks controlled by various companies and states.
At any point along its routed, it may be inspected, or even modified.
(((tampering)))If it is important that something remain secret,
such as the ((password)) to your ((email)) account, or that it arrive
at its destination unmodified, such as the account number you transfer
money to from your bank's website, plain HTTP is not good enough.
indexsee:[Secure HTTP,HTTPS]
(((cryptography)))(((encryption)))The secure ((HTTP)) protocol, whose
((URL))s start with _https://_, wraps HTTP traffic in a way that makes
it harder to read and tamper with. First, the client verifies that the
server is who it claims to be, by requiring it to prove that is has a
cryptographic ((certificate)) issued by a certificate authority that
the ((browser)) recognizes. Next, all data going over the
((connection)) is encrypted in a way that should prevent eavesdropping
and tampering.
Thus, when it works right, ((HTTPS)) prevents both the situation where
someone impersonates the website you were trying to talk to, and the
situation where someone is snooping on your communication. It is not
perfect, and there have been various incidents where HTTPS failed due
to forged or stolen certificates and broken software. Still, plain
HTTP is trivial to mess with, whereas breaking HTTPS requires the kind
of effort that only states or sophisticated criminal organizations can
hope to make.
== Summary ==
In this chapter, we saw that HTTP is a protocol for accessing
resources over the Internet. A _client_ sends a request, which
contains a method (usually `GET`) and a path that identifies a
resource. The _server_ then decides what to do with the request, and
responds with a status code and a response body. Both requests and
responses may contain headers, providing additional information.
Browsers make `GET` requests to fetch the resources needed to display
a web page. A web page may also contain forms, which allow information
entered by the user to be sent along in the request made when the form
is submitted. More on that in the link:18_forms.html#forms[next
chapter].
The interface through which browser JavaScript can make HTTP requests
is called `XMLHttpRequest`. You can usually ignore the “XML” part of
that name (but you still have to type it). There are two ways in which
it can be used—synchronous, which blocks everything until the request
finishes, and asynchronous, which requires an event handler to notice
that the response came in. In almost all cases, asynchronous is
preferable. Making a request looks like this:
[source,javascript]
----
var req = new XMLHttpRequest();
req.open("GET", "example/data.txt", true);
req.addEventListener("load", function() {
console.log(req.statusCode);
});
req.send(null);
----
Asynchronous programming is tricky. _Promises_ are an interface that
makes it slightly easier, by helping route error conditions and
exceptions to the right handler, and abstracting away some of the more
repetetive and error-prone elements in this style of programming.
== Exercises ==
[[exercise_accept]]
=== Content negotiation ===
(((Accept header)))(((media type)))(((document format)))(((content
negotiation (exercise))))One of the things that HTTP can do, but which
we have not discussed in this chapter yet, is called _content
negotiation_. The `Accept` header for a request can be used to tell
the server what type of document the client would like to get. Many
servers ignore this header, but when a server knows of various ways to
encode a resource, it can look at this header and send the one that
the client prefers.
(((media type)))(((MIME type)))The URL
http://eloquentjavascript.net/author[_eloquentjavascript.net/author_]
is configured to respond with either plain text, HTML, or JSON,
depending on what the client asks for. These formats are identified by
the standardized _media types_ `text/plain`, `text/html`, and
`application/json`.
(((setRequestHeader method)))(((XMLHttpRequest)))Send requests to
fetch all three formats of this resource. Use the `setRequestHeader`
method of your `XMLHttpRequest` object set the header named `Accept`
to one of the media types given above. Make sure you set the header
_after_ calling `open`, but before calling `send`.
Finally, try asking for the media type `application/rainbows+unicorns`
and see what happens.
ifdef::html_target[]
// test: no
[source,javascript]
----
// Your code here.
----
endif::html_target[]
!!solution!!
(((synchronous I/O)))(((content negotiation (exercise))))See the
various examples of using an `XMLHttpRequest` in this chapter for an
example of the method calls involved in making a request. You can use
a synchronous request (by setting the third parameter to `open` to
`false`) if you want.
(((406 (HTTP status code))))(((Accept header)))Asking for a bogus
media type will return a response with code 406 “Not acceptable”,
which is the code a server should return when it can not fulfill the
`Accept` header.
!!solution!!
=== Waiting for multiple promises ===
(((all function)))(((Promise constructor)))The `Promise` constructor
has an `all` method which, given an array of ((promise))s, returns a
promise that waits for all of them to finish, and then succeeds
itself, yielding an array of result values. If any of the promises in
the array fail, the promise returned by `all` fails too (with the
failure value from the failing promise).
Try to implement something like this yourself, as a regular function
called `all`.
Note that after a promise is resolved (has succeeded or failed), it
can not succeed or fail again, and further calls to the functions that
resolve it are ignored. This can simplify the way you handle failure
of your promise.
ifdef::html_target[]
// test: no
[source,javascript]
----
function all(promises) {
return new Promise(function(success, fail) {
// Your code here.
});
}
// Test code.
all([], function(array) {
console.log("This should be []:", array);
});
function soon(val) {
return new Promise(function(success) {
setTimeout(function() { success(val); },
Math.random() * 500);
});
}
all([soon(1), soon(2), soon(3)]).then(function(array) {
console.log("This should be [1, 2, 3]:", array);
});
function fail() {
return new Promise(function(success, fail) {
fail(new Error("boom"));
});
}
all([soon(1), fail(), soon(3)]).then(function(array) {
console.log("We should not get here");
}, function(error) {
if (error.message != "boom")
console.log("Unexpected failure:", error);
});
----
endif::html_target[]
!!solution!!
(((all function)))(((Promise constructor)))(((then method)))The
function passed to the `Promise` constructor will have to call `then`
on each of the promises in the given array. When one of them succeeds,
two things need to happen—the resulting value needs to be stored in
the correct position of a result array, and we must check whether this
was the last pending ((promise)), and finish our own promise when it
was.
(((counter variable)))The latter can be done with a counter, which is
initialized to the length of the input array, and which we subtract
one for every time a promise succeeds. When it reaches zero, we are
done. Make sure you take the situation where the input array is empty
(and thus no promise will ever resolve) into account.
Handling failure requires some though, but turns out to be extremely
simple. Just pass the failure function of the wrapping promise to each
of the promises in the array, so that a failure in one of them
triggers failure of the whole wrapper.
!!solution!!