Firefox Hacks [Electronic resources] نسخه متنی

Hack 47. Update Browser Detection Scripts

Work out whether your web page is loaded into
Firefox.

There are all kinds of Mozilla
browsers and even different variants
of Firefox [Hack #92], although
the standard distribution of Firefox is by far
the most popular distribution. This
hack explains how to detect Firefox. Your web page can also make a
display contract with Firefox [Hack #46] .

5.5.1. Detect Firefox by UserAgent

Either behind the web
server or in the web page, the foremost
identifier of Firefox is the browser user agent (UA)
string. The UA string is defined in RFC 1945 and RFC 2068. It is sent
over the wire from web browser to web server in this header line:

User-Agent: string

It appears in the Apache server environment as this environment
variable:

HTTP_USER_AGENT=string

It appears in JavaScript as this host object property:

window.navigator.userAgent

For quick sanity checks, the user agent for Firefox can also be
displayed using the HelpAbout Mozilla Firefox menu item.
That page can be displayed separately using this URL:

chrome://browser/content/aboutDialog.xul

Select the text of the user-agent string with the mouse and drag up
or down if it looks odd when it first appears.

To detect Firefox, chop up the UA string. Useful JavaScript functions
include these:

userAgent.substring(start, length); // returns a fixed substring
userAgent.split(" ");               // returns array of whitespace-separated pieces
userAgent.match(/.*fox/i);          // returns case-insensitive regexp match

Here are the meaningful substrings of the user agent:

Mozilla

The user agent is probably a web browser.

Gecko

The user agent contains Mozilla Gecko display technology, a big part
of Firefox. Gecko guarantees that web pages will look the same across
all Mozilla browsers and that quality web standards support is
present. Browsers such as Galeon and Camino also fall into this
category; however, they don't support Firefox
extensions.

Firefox

The user agent is a Firefox browser, with full extension support and
other niceties that are implied by Firefox.

Firebird, Phoenix

The user agent is a very old version of Firefox. It is best to treat
it as a non-Firefox browser, because the version is too early to
support some modern Firefox features.

en-US (or other strings in the same spot)

The menus, status bar messages, and other browser text appear to the
user in U.S. English. Don't write alerts or other
messages in the wrong language.

5.5.2. Detect Firefox via the DOM

The user agent string is exposed to the Document Object Model (DOM)
of a given web page as described earlier in this hack. Other
detection methods also exist in the
DOM.

5.5.2.1 document.all

Testing this DOM property like this is a common way of detecting
Internet Explorer (IE):

if (document.all ) {  }

Some web pages are built in ignorance of the constraints of the
global Web. Such pages use document.all without
first checking to see if it exists. These cases, such as the
following example, cause pages displayed in non-IE web browsers to
fail:

if ( document.all("id") == null ) {  }

Firefox has special support for document.all, as
follows:

If the all property is stated in a Boolean
(true/false) context, as in the first example, then Firefox (and all
recent Mozilla-based browsers) will report that the method
doesn't exist. This is the same as matters have
always been.

If the all property is invoked as a method, as in
the second example, then Firefox (and all recent Mozilla-based
browsers) will behave the same way as IE. This preserves the display
of some old, ignorant web pages.

This support only exists when a web page is displayed in
Quirks mode. Quirks mode is for old, legacy HTML
documents. One easy way to ensure that Quirks mode is operating is to
leave out an HTML document's
<!DOCTYPE> declaration. You can see if a
page uses Quirks mode or not using ToolsPage
InfoGeneral. Look at the Render Mode line item for the
current mode.

5.5.2.2 document.implementation.hasFeature(type,version)

This method is defined in the W3C's DOM 2 Core
standard. It allows a script to test for UA support for specific
standards. For example, this test can be used to test for CSS2
support:

document.implementation.hasFeature("CSS2","2.0");

In particular, this method can be used to test whether the Firefox UA
is built so that it includes SVG:

document.implementation.hasFeature("org.w3c.svg","dummy string");

It can also be used to report whether specific SVG
features (a term specific to SVG) are implemented. To do
so, provide an SVG 1.1 feature URL as the first argument, like this:

"http://www.w3.org/TR/SVG11/feature#Structure"

h5

This DOM 0 collection, present in Netscape 4.x, is not present in
Firefox at all. It's unusable as a positive test of
Firefox, so it no longer stands for "Netscape 4.x or
higher." Now it stands for
"Netscape 4.x only."

5.5.3. Detect JavaScript Version Support

All Firefox browsers support at least
JavaScript Version 1.5. That version
fully supports ECMAScript Version 1, Edition 3. Firefox also supports
earlier versions of JavaScript, but you should never use them.
Include external scripts within HTML and XHTML with this tag:

<script type="text/javascript" src="></script>

Similarly, include embedded scripts with this tag:

<script type="text/javascript">
content 
</script>

Support all scripts used strictly in XUL with this tag:

<script type="application/x-javascript" src=" />

See also [Hack #74] for some fancy
new features.