Nullcoder

Count lines of Code in a project :

find . -type f -name "*.php" -exec cat {} \; | wc

AuthSub in the Google Data Protocol Client Libraries

http://code.google.com/intl/fr/apis/gdata/docs/auth/authsub.html

Registration for Web-Based Applications

http://code.google.com/intl/fr/apis/accounts/docs/RegistrationForWebAppsAuto.html

Manage your domains

https://www.google.com/accounts/ManageDomains

Generate SSL key (optionnal)

http://code.google.com/intl/fr/apis/gdata/docs/auth/authsub.html#Registered

HowTo Google Calendar API : PHP (Part 1)

Google Calendar API request/response works in the form of Google Data API feeds. The popular PHP client library used to work with Calendar service is Zend's GData which is also distributed as a part of Zend Framework.

Install GData

First download the latest release GData library from here. Extract it and keep the folder anywhere you like. Lets assume that we kept it at /home/abbas/ZendGdata/.

Next step is to make sure you can include ZendGdata/library in your php script. This is done by setting include_path directive and this can be done in the following three ways:

   * Permanently set the include_path directive in your php.ini configuration file from the command line
   * Set the include_path path variable on a "per directory" level using .htaccess
   * Use the set_include_path() function to dynamically set the include path in your scripts

We will use the set_include_path() function to set the include path. The code will look something like this...

$path = '/home/abbas/ZendGdata/library';
// Append the library path to existing paths
$oldPath = set_include_path(get_include_path() . PATH_SEPARATOR . $path);

Authentication

The Gdata libarary can be used to access both public as well as private calendars. For private calendars we need to authenticate to the calendar servers. There are three types of authentication viz AuthSub proxy authentication, ClientLogin username/password authentication and Magic cookie authentication. We will be using ClientLogin username/password authentication in this tutorial as it is much easier to use and fits in the example we are going to take. Here's how to authenticate using username/password (Google calendar account credentials):

$path = '/home/abbas/ZendGdata/library';
$oldPath = set_include_path(get_include_path() . PATH_SEPARATOR . $path);
require_once 'Zend/Loader.php';
Zend_Loader::loadClass('Zend_Gdata');
Zend_Loader::loadClass('Zend_Gdata_ClientLogin');
Zend_Loader::loadClass('Zend_Gdata_Calendar');
// User whose calendars you want to access
$user = 'username@gmail.com';
$pass = 'yourpass';
$service = Zend_Gdata_Calendar::AUTH_SERVICE_NAME; // predefined service name for calendar
$client = Zend_Gdata_ClientLogin::getHttpClient($user, $pass, $service);

At this point authentication token is retrieved and $client is an object of Zend_Http_Client with appropriate Authentication header.

Retrieving list of Calendars

Now lets use the http client object to retrieve a list of all calanders. (In continuation of above code..)

$service = new Zend_Gdata_Calendar($client);
// Get the calendar list feed
$listFeed = $service->getCalendarListFeed();
echo "<h1>Calendar List</h1>";
echo "<ul>";
foreach ($listFeed as $calendar) {
	echo "<li>" . $calendar->title . " (" . $calendar->id . ")</li>";
}
echo "</ul>";

HowTo Google Calendar API : PHP – Retrieve events (Part 2)

Continuing the same topic lets now see how to retrieve events from Google calendar using GData library.

Firstly, lets instantiate the Zend_Gdata_Calendar class.

$path = '/home/abbas/ZendGdata/library';
$oldPath = set_include_path(get_include_path() . PATH_SEPARATOR . $path);
require_once 'Zend/Loader.php';
Zend_Loader::loadClass('Zend_Gdata');
Zend_Loader::loadClass('Zend_Gdata_ClientLogin');
Zend_Loader::loadClass('Zend_Gdata_Calendar');
// User whose calendars you want to access
$user = 'username@gmail.com';
$pass = 'yourpass';
$serviceName = Zend_Gdata_Calendar::AUTH_SERVICE_NAME; // predefined service name for calendar
$client = Zend_Gdata_ClientLogin::getHttpClient($user, $pass, $serviceName);
$service = new Zend_Gdata_Calendar($client);

For retrieving events, specially constructed URLs are used and Zend_Gdata_Calendar_EventQuery makes it easy to construct URL based on the parameters passed to it. Following three parameters are important.

   * User: If not specified, the "default" (currently authenticated) user is used i.e. default calendar. Use email address for shared calendars or use the Calendar ID
   * Visibility: Whether user's public or private calendars should be searched
   * Projection: Specifies the amount of data that should be returned. Generally use "full".

Retrieve all events

Lets get a list of events from default calendar.

$query = $service->newEventQuery();
// Set different query parameters
$query->setUser('default');
$query->setVisibility('private');
$query->setProjection('full');
$query->setOrderby('starttime');
// Get the event list
try {
	$eventFeed = $service->getCalendarEventFeed($query);
} catch (Zend_Gdata_App_Exception $e) {
	echo "Error: " . $e->getMessage();
}
echo "<ul>";
foreach ($eventFeed as $event) {
	echo "<li>" . $event->title . " </li>";
}
echo "</ul>";

Retrieve events occuring between a date range

Just add these two parameters to the query before getting the event feed

// Start date from where to get the events
$query->setStartMin('2010-02-01');
// End date
$query->setStartMax('2010-03-15');

Retrieving individual events

This can be done in two ways. If you know the event ID then set the event parameter in the query and use getCalendarEventEntry method instead of getCalendarEventFeed.

$query->setEvent('pomb7gnnum6sjiii6qgb76xyz');
// Get the individual event
try {
	$event = $service->getCalendarEventEntry($query);
} catch (Zend_Gdata_App_Exception $e) {
	echo "Error: " . $e->getMessage();
}

or if you know the full event URL (you get this when you retrieve all events), you can skip creating the query object and directly pass the event URL to getCalendarEventEntry.

$eventURL = "http://www.google.com/calendar/feeds/default/private/full/pomb7gnnum6sjiii6qgb76xyz";
try {
	$event = $service->getCalendarEventEntry($eventURL);
} catch (Zend_Gdata_App_Exception $e) {
	echo "Error: " . $e->getMessage();
}

HowTo Google Calendar API : PHP – Manage events (Part 3)

Now its time to manage events i.e. create, update and delete events.

First we need the Calendar service instance, so lets instantiate the Zend_GData_Calendar class.

$path = '/home/abbas/ZendGdata/library';
$oldPath = set_include_path(get_include_path() . PATH_SEPARATOR . $path);
require_once 'Zend/Loader.php';
Zend_Loader::loadClass('Zend_Gdata');
Zend_Loader::loadClass('Zend_Gdata_ClientLogin');
Zend_Loader::loadClass('Zend_Gdata_Calendar');
// User whose calendars you want to access
$user = 'username@gmail.com';
$pass = 'yourpass';
$serviceName = Zend_Gdata_Calendar::AUTH_SERVICE_NAME; // predefined service name for calendar
$client = Zend_Gdata_ClientLogin::getHttpClient($user, $pass, $serviceName);
$service = new Zend_Gdata_Calendar($client);

Creating Single Occurrence Events

Lets directly look at the code which has self explanatory comments

// Create a new event object using calendar service's factory method.
// We will then set different attributes of event in this object.
$event= $service->newEventEntry();
// Create a new title instance and set it in the event
$event->title = $service->newTitle("Some Event");
// Where attribute can have multiple values and hence passing an array of where objects
$event->where = array($service->newWhere("Nagpur, India"));
$event->content = $service->newContent("Some event content.");
// Create an object of When and set start and end datetime for the event
$when = $service->newWhen();
// Set start and end times in RFC3339 (http://www.ietf.org/rfc/rfc3339.txt)
$when->startTime = "2010-07-08T16:30:00.000+05:30"; // 8th July 2010, 4:30 pm (+5:30 GMT)
$when->endTime = "2010-07-08T17:30:00.000+05:30"; // 8th July 2010, 5:30 pm (+5:30 GMT)
// Set the when attribute for the event
$event->when = array($when);
// Create the event on google server
$newEvent = $service->insertEvent($event);
// URI of the new event which can be saved locally for later use
$eventUri = $newEvent->id->text;

If you want the event to last the whole day then only set the date in startTime and skip endTime attribute of "when"

$when->startTime = "2010-07-08"; // 8th July 2010, Whole day event

Creating recurring events

Recurring events are created almost similar to non recurring events. The only difference is that instead of "where" attribute we would set "recurrence" attribute for the event. Recurrence attribute is a string pattern in iCalendar standard RFC2445. For more details please check out Google Data API Guide.

// This event will occur all day every Friday starting from 9th July 2010 until 8th Dec 2015
$recurrence = "DTSTART;VALUE=DATE:20100709\r\n" .
		"DTEND;VALUE=DATE:20100710\r\n" .
		"RRULE:FREQ=WEEKLY;BYDAY=Fr;UNTIL=20151208\r\n";
$event->recurrence = $service->newRecurrence($recurrence);

Updating an event

For modifying an event, we first need to get its reference. This can be done using the event URI which we got after we created the event (see above). Once we get the event reference, we can pretty much change any attribute of it (as we did while creating an event) and then we have to save it.

// URI of the event which we got after creating it.
$eventUri = "http://www.google.com/calendar/feeds/default/private/full/53608ibmrtnb57o7hqf8l1tsu4";
// Get the event
$event = $service->getCalendarEventEntry($eventUri);
// Change the title
$event->title = $service->newTitle("New Title!");
// Save the event
$event->save();

Deleting an event

Deleting an event is very simple. Just get the event reference (as we did for updating) and call the event's delete method.

// Get the event
$event = $service->getCalendarEventEntry($eventUri);
// Delete the event
$event->delete();

Remove the kernel metapackage that pulls in each new kernel :

sudo aptitude remove linux-generic linux-headers-generic linux-image-generic

This will stop the kernel updating, it won't remove any kernels. Now newer kernels must be update manually.

sudo aptitude install cryptsetup dmsetup

Erase the partition with random writing

sudo aptitude install e2fsprogs
sudo /sbin/badblocks -s -w -t random -v /dev/sdxx

Formating the partition with luks

sudo luksformat -t ext2 /dev/sdxx
mkdir /media/cryptfs

Add to /etc/fstab : /dev/mapper/cdisk1 /media/cryptfs ext2 noauto,defaults 0 0

Manually mount the partition

cryptsetup luksOpen /dev/sdxx cdisk1

Importing/Creating a new git project

sudo aptitude install git-core

mkdir myapp
cd myapp
tar xzf myapp.tar.gz
git config --global user.name "Your Name Comes Here"
git config --global user.email you@yourdomain.example.com
git init

you may notice a new directory created, named ".git".

Next, tell git to take a snapshot of the contents of all files under the current directory :

git add .

This snapshot is now stored in a temporary staging area which git calls the "index". You can permanently store the contents of the index in the repository with git commit:

git commit

Installing Java

sudo apt-get install sun-java6-jdk

Select the /usr/lib/jvm/java-6-sun/jre/bin/java alternative

sudo update-alternatives --config java

To start, CruiseControl needs the JAVA_HOME variable set. To set it add the lone below to your /etc/environment.

JAVA_HOME="/usr/lib/jvm/java-6-sun"

Installing CruiseControl

Install CruiseControl: unzip the bin release to a directory. For example

sudo unzip cruisecontrol-bin-2.8.3.zip /opt
cd /opt/cruisecontrol-bin-2.8.3
sudo ln -s cruisecontrol-bin-2.8.3 cruisecontrol

Init script for CruiseControl

The script implements the following functions: start, stop, restart, status. Save it to /etc/init.d/cruisecontrol and make it executable (chmod +x /etc/init.d/cruisecontrol).

#!/bin/sh
### BEGIN INIT INFO
# Provides:          cruisecontrol
# Required-Start:    $local_fs $remote_fs $network $syslog
# Required-Stop:     $local_fs $remote_fs $network $syslog
# Default-Start:     2 3 4 5
# Default-Stop:      0 1 6
# Short-Description: Start/stop cruisecontrol CI server
### END INIT INFO
#
# cruisecontrol         This init.d script is used to start cruisecontrol.
#                       It basically just calls cruisecontrol.sh.

# ENV="env -i LANG=C PATH=/usr/local/bin:/usr/bin:/bin"

CCON_PATH=/opt/cruisecontrol
CCON_USER=www-data
PIDFILE=$CCON_PATH/cc.pid
LOGFILE=/var/log/cruisecontrol.log
JAVA_HOME=/usr/lib/jvm/java-6-sun
export JAVA_HOME

. /lib/lsb/init-functions

ccon_is_running() {
    if [ -f $PIDFILE ]; then
       PID=`cat $PIDFILE`
       PID_COUNT=`ps aux | grep -E $PID | grep -v grep | wc -l`
       if [ $PID_COUNT = 1 ]; then
         return 1
       fi
    fi

    return 0
}

ccon_start() {
        ccon_is_running
        rc=$?
        if [ $rc -eq 1 ]; then
                log_failure_msg "CruiseControl is already running"
        else
                log_daemon_msg "Starting CI server" "CruiseControl"
                cd $CCON_PATH
                sudo -E -H -u $CCON_USER ./cruisecontrol.sh >> $LOGFILE 2>&1
                log_end_msg $?
        fi
}

ccon_stop() {
        ccon_is_running
        rc=$?
        if [ $rc -eq 1 ]; then
                log_daemon_msg "Stopping CI server" "CruiseControl"
                PID=`cat $PIDFILE`

                retval=0
                i=0
                while $(kill "$PID" 2> /dev/null);  do
                        if [ $i = '60' ]; then
                                echo ""
                                log_failure_msg "CruiseControl is taking too long to shutdown"
                                retval=1
                                break
                        else
                                if [ $i = '0' ]; then
                                        echo -n " ... waiting "
                                else
                                        echo -n "."
                                fi
                                i=$(($i+1))
                                sleep 1
                      fi
                 done

                log_end_msg $retval
        else
                log_failure_msg "CruiseControl is not running"
        fi
}

ccon_status() {
        ccon_is_running
        rc=$?
        if [ $rc -eq 1 ]; then
                PID=`cat $PIDFILE`
                log_success_msg "CruiseControl is running (pid $PID)."
        else
                log_failure_msg "CruiseControl is not running."
        fi

}

case $1 in
        start)
                ccon_start
        ;;
        stop)
                ccon_stop
        ;;
        restart)
                ccon_stop
                ccon_start
        ;;
        status)
                ccon_status
        ;;
        *)
                log_success_msg "Usage: /etc/init.d/cruisecontrol {start|stop|restart|status}"
                exit 1
        ;;
esac

Then change the ownership of the cruisecontrol installation to the user you specified in the init script (in my case www-data):

chown -R www-data.www-data /opt/cruisecontrol-bin-2.8.3/

Additionally you can add it to the default runlevels to start it automatically on system boot.

update-rc.d cruisecontrol defaults

Installing Xdebug

sudo aptitude install php5-xdebug

Or :

sudo aptitude install php5-dev php-pear 
pecl install xdebug

• This will create an extension xdebug.so
• Windows users will instead find the binaries here
• You must add this line to your cli php.ini

zend_extension="FULL_PATH_TO/xdebug.so"

Installing PHP extensions

sudo pear upgrade pear

sudo pear channel-discover pear.phpunit.de
sudo pear channel-discover pear.symfony-project.com
sudo pear install phpunit/PHPUnit

sudo pear channel-discover components.ez.no 
sudo pear install -a ezc/Graph 

sudo pear config-set preferred_state alpha
sudo pear install PHP_CodeSniffer
sudo pear install phpunit/PHP_CodeBrowser
sudo pear config-set preferred_state stable
sudo pear install phpunit/PHP_Timer
sudo pear install phpunit/phpcpd

sudo pear channel-discover pear.pdepend.org
sudo pear install pdepend/PHP_Depend-beta

sudo pear channel-discover pear.phpmd.org
sudo pear install --alldeps phpmd/PHP_PMD-alpha

Installing phpUnderControl

cd /opt
sudo git clone git://github.com/manuelpichler/phpUnderControl.git
sudo phpuc install cruisecontrol

Example projets :

sudo phpuc example /opt/cruisecontrol

Removing a project :

sudo phpuc delete --project-name connectfour /opt/cruisecontrol
sudo phpuc delete --project-name php-under-control /opt/cruisecontrol

Importing Your Project

cd /opt/cruisecontrol
cd projects/myapp

create the build.xml file :

<project name="myapp" default="build" basedir=".">

  <property name="builddir" value="${basedir}/build" />

  <target name="prepare" depends="clean,init" />

  <target name="init">
      <mkdir dir="${builddir}" />
      <mkdir dir="${builddir}/api" />
      <mkdir dir="${builddir}/log" />
      <mkdir dir="${builddir}/coverage" />
      <mkdir dir="${builddir}/phpcb" />
  </target>

    <!--
        Removes all temporary build artifacts
    -->
    <target name="clean">
        <delete dir="${builddir}" />
    </target>


    <target name="checkout">
    <exec dir="${basedir}/source" executable="git">
      <arg line="pull"> </arg>
    </exec>
  </target>

  <!--  Checkout for SVN 
  <target name="checkout">
    <exec executable="svn" dir="${basedir}/source">
      <arg line="up" />
    </exec>
  </target>
  -->

  <target name="phpunit">
    <exec executable="phpunit" dir="${basedir}/source" failonerror="on">
      <arg line="--log-junit ${builddir}/log/junit.xml --coverage-clover ${builddir}/log/phpunit.coverage.xml --coverage-html ${builddir}/coverage phpucAllTests testsuite/all_tests.php" />
    </exec>
  </target> 

  <target name="phpcpd" depends="init">
    <exec executable="phpcpd" failonerror="false">
      <arg line="--log-pmd ${builddir}/log/pmd-cpd.xml ${basedir}/source --exclude plugin/,test/,view/,webroot/" />
      </exec>
  </target>

  <target name="php-documentor">
    <exec executable="phpdoc" dir="${basedir}/source">
      <arg line="-ct type -ue on -t ${builddir}/api -tb '/usr/share/php/data/phpUnderControl/data/phpdoc' -o HTML:Phpuc:phpuc -d . -i plugin/,test/,webroot/,view/"/>
    </exec>
  </target>

  <target name="php-codesniffer">
    <exec executable="phpcs" dir="${basedir}/source" output="${builddir}/log/checkstyle.xml" error="/tmp/checkstyle.error.log">
      <arg line="--report=checkstyle --standard=PEAR --ignore=plugin/,view/,test/,webroot/ ."/>
    </exec>
  </target>

  <target name="phpmd">
    <exec executable="phpmd" dir="${basedir}/source">
      <arg line=". xml codesize,unusedcode,naming --reportfile ${builddir}/log/pmd.xml --exclude plugin/,test/,view/,webroot/"/>
    </exec>
  </target> 

  <target name="build" depends="checkout,php-documentor,php-codesniffer,phpmd,phpcpd,phpunit"/>
</project>

Init & import Git / SVN (choose one solution)

ant init
sudo git clone file:///opt/src/mylocalgitrepository/myapp.git source

Other possible import solution :

sudo svn co svn://my.svnserver.com/myproject/trunk source
sudo git clone git://my.gitserver.com/myrepository/myapp.git source

Configure CruiseControl

<cruisecontrol>
  <project name="....">
    ...
  </project>
     ...

  <project name="myapp" buildafterfailed="false">
    <plugin name="git" classname="net.sourceforge.cruisecontrol.sourcecontrols.Git"/>

    <!-- SVN plugin
    <plugin name="svn" classname="net.sourceforge.cruisecontrol.sourcecontrols.SVN" />
    -->

    <listeners>
      <currentbuildstatuslistener file="logs/${project.name}/status.txt"/>
    </listeners>

    <modificationset quietperiod="60">
      <git localworkingcopy="projects/${project.name}/source/"/>
    </modificationset>

    <schedule interval="300">
      <ant anthome="apache-ant-1.7.0" buildfile="projects/${project.name}/build.xml"/>
    </schedule>

    <log dir="logs/${project.name}">
      <merge dir="projects/${project.name}/build/logs/"/>
    </log>

    <publishers>
      <artifactspublisher subdirectory="api" dest="artifacts/${project.name}" dir="projects/${project.name}/build/api"/>
      <artifactspublisher subdirectory="coverage" dest="artifacts/${project.name}" dir="projects/${project.name}/build/coverage"/>
      <execute command="phpcb --log projects/${project.name}/build/log --output projects/${project.name}/build/phpcb"/>
      <artifactspublisher dir="projects/${project.name}/build/phpcb" dest="artifacts/${project.name}" subdirectory="php-code-browser"/>
      <execute command="phpuc graph logs/${project.name} artifacts/${project.name}"/>

      <!-- Mailing
      <email mailhost="smtp.localhost"
             username="username"
             password="password"
             returnaddress="cruise@phpundercontrol.org"
             buildresultsurl="http://localhost:8080/buildresults/${project.name}"
             skipusers="true"
             spamwhilebroken="true">
        <always address="successful@maildomain.com"/>
        <always address="cruise@maildomain.com"/>
        <failure address="failed@maildomain.com"/>
      </email>
      -->

    </publishers>

  </project>

</cruisecontrol>

Purge unused configuration file :

sudo dpkg --purge $(COLUMNS=200 dpkg -l | grep "^rc" | tr -s ' ' | cut -d ' ' -f 2)

or for on packet :

sudo aptitude remove nom-du-paquet --purge

There are many great tools for backing up or synchronizing directories :

• duplicity
• unison
• rdiff_backup
• rsync

Your system is configured properly, but each time you plug your USB Mass Storage device you need to check all the "/dev/sdxx". Using udev you can make sure that, no matter what order you plug in your USB devices, the storage device is always at /dev/usbdrive (or whatever mount point you like).

First, find the ID of your usb drive by using /sys: (assuming the device is currently in /dev/sdc)

#> udevinfo -a -p `udevinfo -q path -n /dev/sdc`

If your device is an USB device you should have a section like the following :

 looking at parent device '/devices/pci0000:00/0000:00:1d.7/usb7/7-4':
    KERNELS=="7-4"
    SUBSYSTEMS=="usb"
    DRIVERS=="usb"
    ATTRS{dev}=="189:776"
    ATTRS{configuration}=="USB Mass Storage"
    ATTRS{bNumInterfaces}==" 1"
    ATTRS{bConfigurationValue}=="1"
    ATTRS{bmAttributes}=="c0"
    ATTRS{bMaxPower}=="  2mA"
    ATTRS{urbnum}=="14451"
    ATTRS{idVendor}=="059f"
    ATTRS{idProduct}=="1021"
    ATTRS{bcdDevice}=="0001"
    ATTRS{bDeviceClass}=="00"
    ATTRS{bDeviceSubClass}=="00"
    ATTRS{bDeviceProtocol}=="00"
    ATTRS{bNumConfigurations}=="1"
    ATTRS{bMaxPacketSize0}=="64"
    ATTRS{speed}=="480"
    ATTRS{busnum}=="7"
    ATTRS{devnum}=="9"
    ATTRS{version}==" 2.00"
    ATTRS{maxchild}=="0"
    ATTRS{quirks}=="0x0"
    ATTRS{authorized}=="1"
    ATTRS{manufacturer}=="LaCie"
    ATTRS{product}=="little disk"
    ATTRS{serial}=="00E001036060D"

ATTRS{serial}=="00E001036060D" seems to be a good values to identifying the device.

On this device i've two partitions : - /dev/sdc1 (fat32) - /dev/sdc2 (ext2fs)

and i want them to be mounted on /media/laciewin ans /media/lacielinux respectively.

So next, you'll have add the following rules in your "/etc/udev/rules.d/10-local.rules" files (create it if you don't have one):

SUBSYSTEMS=="usb",ATTRS{serial}=="00E001036060D",KERNEL=="sd?1", NAME="laciewin", SYMLINK="usbdevices/laciewin"

SUBSYSTEMS=="usb",ATTRS{serial}=="00E001036060D",KERNEL=="sd?2", NAME="lacielinux", SYMLINK="usbdevices/lacielinux"

Then make udevd re-read the updated ruleset:

#> sudo udevcontrol reload_rules

or:

#> sudo /etc/init.d/udev restart

Finally, you'll have to create the mount point and configuring /etc/fstab :

#> sudo mkdir /media/laciewin
#> sudo mkdir /media/lacielinux

Add the following rules in your /etc/fstab :

/dev/laciewin  /media/laciewin  vfat  defaults,user,utf8,umask=007,gid=46 0       1
/dev/lacielinux  /media/lacielinux   ext2    defaults,user        0       2

Should work !

Usefull infos for calling parent Javascript function (e.g from inside an iframe)

window.top : Returns a reference to the topmost window in the window hierarchy.

window.parent : Returns a reference to the parent of the current window or subframe.

window.opener : Returns a reference to the window that opened this current window.

function parentCall()
  {
     if(typeof(window.parent)!="undefined"))
       {
           window.parent.myFunction();
       }
  }

In JavaScript null is an object. undefined is used for things that don't exist. The DOM returns null for almost all cases where it fails to find some structure in the document, but in JavaScript itself undefined is the value used.

For example :

     (name is undefined)

You: What is name?

JavaScript: name? What's a name? I don't know what you're talking about.

    name = null;

You: What is name?

JavaScript: I don't know.

so null and undefined are not strictly equivalent. If you really want to check for null, do:

    if (varname === null)

If you want to check if a variable exist

    if (typeof(varname) != 'undefined')