Java 'The Next Generation': JS Advanced

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Friday, 15 April 2011

Javascript Summary

JavaScript Summary

This tutorial has taught you how to add JavaScript to your HTML pages, to make your web site more dynamic and interactive.
You have learned how to create responses to events, validate forms and how to make different scripts run in response to different scenarios.
You have also learned how to create and use objects, and how to use JavaScript's built-in objects.
For more information on JavaScript, please look at our JavaScript examples and our JavaScript reference.

Now You Know JavaScript, What's Next?

The next step is to learn about the HTML DOM, jQuery, and AJAX.
If you want to learn about server-side scripting, the next step is to learn ASP or PHP.
HTML DOM
The HTML DOM defines a standard way for accessing and manipulating HTML documents.
The HTML DOM is platform and language independent and can be used by any programming language like Java, JavaScript, and VBScript.
If you want to learn more about the DOM, please visit our HTML DOM tutorial.
jQuery
jQuery is a JavaScript Library.
jQuery greatly simplifies JavaScript programming.
If you want to learn more about jQuery, please visit our jQuery tutorial.
AJAX
AJAX = Asynchronous JavaScript and XML.
AJAX is not a new programming language, but a new way to use existing standards.
AJAX is about exchanging data with a server, and update parts of a web page - without reloading the whole page.
Examples of applications using AJAX: Google Maps, Gmail, Youtube, and Facebook tabs.
If you want to learn more about AJAX, please visit our AJAX tutorial.
ASP / PHP
While scripts in an HTML file are executed on the client (in the browser), scripts in an ASP/PHP file are executed on the server.
With ASP/PHP you can dynamically edit, change or add any content of a Web page, respond to data submitted from HTML forms, access any data or databases and return the results to a browser, customize a Web page to make it more useful for individual users.
Since ASP/PHP files are returned as plain HTML, they can be viewed in any browser.

Create Own Object

JavaScript Objects

Earlier in this tutorial we have seen that JavaScript has several built-in objects, like String, Date, Array, and more. In addition to these built-in objects, you can also create your own.
An object is just a special kind of data, with a collection of properties and methods.
Let's illustrate with an example: A person is an object. Properties are the values associated with the object. The persons' properties include name, height, weight, age, skin tone, eye color, etc. All persons have these properties, but the values of those properties will differ from person to person. Objects also have methods. Methods are the actions that can be performed on objects. The persons' methods could be eat(), sleep(), work(), play(), etc.

Properties

The syntax for accessing a property of an object is:

objName.propName

You can add properties to an object by simply giving it a value. Assume that the personObj already exists - you can give it properties named firstname, lastname, age, and eyecolor as follows:

personObj.firstname="John";
personObj.lastname="Doe";
personObj.age=30;
personObj.eyecolor="blue";

document.write(personObj.firstname);

The code above will generate the following output:

John

Methods

An object can also contain methods.
You can call a method with the following syntax:

objName.methodName()

Note: Parameters required for the method can be passed between the parentheses.
To call a method called sleep() for the personObj:

personObj.sleep();

Creating Your Own Objects

There are different ways to create a new object:
1. Create a direct instance of an object
The following code creates an new instance of an object, and adds four properties to it:

personObj=new Object();
personObj.firstname="John";
personObj.lastname="Doe";
personObj.age=50;
personObj.eyecolor="blue";

alternative syntax (using object literals):

personObj={firstname:"John",lastname:"Doe",age:50,eyecolor:"blue"};

Adding a method to the personObj is also simple. The following code adds a method called eat() to the personObj:

personObj.eat=eat;

2. Create an object constructor
Create a function that construct objects:

function person(firstname,lastname,age,eyecolor)
{
this.firstname=firstname;
this.lastname=lastname;
this.age=age;
this.eyecolor=eyecolor;
}

Inside the function you need to assign things to this.propertyName. The reason for all the "this" stuff is that you're going to have more than one person at a time (which person you're dealing with must be clear). That's what "this" is: the instance of the object at hand.
Once you have the object constructor, you can create new instances of the object, like this:

var myFather=new person("John","Doe",50,"blue");
var myMother=new person("Sally","Rally",48,"green");

You can also add some methods to the person object. This is also done inside the function:

function person(firstname,lastname,age,eyecolor)
{
this.firstname=firstname;
this.lastname=lastname;
this.age=age;
this.eyecolor=eyecolor;

this.newlastname=newlastname;
}

Note that methods are just functions attached to objects. Then we will have to write the newlastname() function:

function newlastname(new_lastname)
{
this.lastname=new_lastname;
}

The newlastname() function defines the person's new last name and assigns that to the person. JavaScript knows which person you're talking about by using "this.". So, now you can write: myMother.newlastname("Doe").

Timing Events

JavaScript Timing Events
With JavaScript, it is possible to execute some code after a specified time-interval. This is called timing events.
It's very easy to time events in JavaScript. The two key methods that are used are:

setTimeout() - executes a code some time in the future
clearTimeout() - cancels the setTimeout()

Note: The setTimeout() and clearTimeout() are both methods of the HTML DOM Window object.

The setTimeout() Method

Syntax

var t=setTimeout("javascript statement",milliseconds);

The setTimeout() method returns a value. In the syntax defined above, the value is stored in a variable called t. If you want to cancel the setTimeout() function, you can refer to it using the variable name.
The first parameter of setTimeout() can be a string of executable code, or a call to a function.
The second parameter indicates how many milliseconds from now you want to execute the first parameter.
Note: There are 1000 milliseconds in one second.

Example

When the button is clicked in the example below, an alert box will be displayed after 3 seconds.

Example

<html>
<head>
<script type="text/javascript">
function timeMsg()
{
var t=setTimeout("alertMsg()",3000);
}
function alertMsg()
{
alert("Hello");
}
</script>
</head>

<body>
<form>
<input type="button" value="Display alert box in 3 seconds"
onclick="timeMsg()" />
</form>
</body>
</html>

Example - Infinite Loop

To get a timer to work in an infinite loop, we must write a function that calls itself.
In the example below, when a button is clicked, the input field will start to count (for ever), starting at 0.
Notice that we also have a function that checks if the timer is already running, to avoid creating additional timers, if the button is pressed more than once:

Example

<html>
<head>
<script type="text/javascript">
var c=0;
var t;
var timer_is_on=0;

function timedCount()
{
document.getElementById('txt').value=c;
c=c+1;
t=setTimeout("timedCount()",1000);
}

function doTimer()
{
if (!timer_is_on)
{
timer_is_on=1;
timedCount();
}
}
</script>
</head>

<body>
<form>
<input type="button" value="Start count!" onclick="doTimer()">
<input type="text" id="txt" />
</form>
</body>
</html>

The clearTimeout() Method

Syntax

clearTimeout(setTimeout_variable)

Example

The example below is the same as the "Infinite Loop" example above. The only difference is that we have now added a "Stop Count!" button that stops the timer:

Example

<html>
<head>
<script type="text/javascript">
var c=0;
var t;
var timer_is_on=0;

function timedCount()
{
document.getElementById('txt').value=c;
c=c+1;
t=setTimeout("timedCount()",1000);
}

function doTimer()
{
if (!timer_is_on)
{
timer_is_on=1;
timedCount();
}
}

function stopCount()
{
clearTimeout(t);
timer_is_on=0;
}
</script>
</head>

<body>
<form>
<input type="button" value="Start count!" onclick="doTimer()">
<input type="text" id="txt">
<input type="button" value="Stop count!" onclick="stopCount()">
</form>
</body>
</html>

Javascript Form Validation

JavaScript Form Validation

JavaScript can be used to validate data in HTML forms before sending off the content to a server.
Form data that typically are checked by a JavaScript could be:

has the user left required fields empty?
has the user entered a valid e-mail address?
has the user entered a valid date?
has the user entered text in a numeric field?

Required Fields

The function below checks if a field has been left empty. If the field is blank, an alert box alerts a message, the function returns false, and the form will not be submitted:

function validateForm()
{
var x=document.forms["myForm"]["fname"].value
if (x==null || x=="")
{
alert("First name must be filled out");
return false;
}
}

The function above could be called when a form is submitted:

Example

<form name="myForm" action="demo_form.asp" onsubmit="return validateForm()" method="post">
First name: <input type="text" name="fname">
<input type="submit" value="Submit">
</form>

E-mail Validation

The function below checks if the content has the general syntax of an email.
This means that the input data must contain an @ sign and at least one dot (.). Also, the @ must not be the first character of the email address, and the last dot must be present after the @ sign, and minimum 2 characters before the end:

function validateForm()
{
var x=document.forms["myForm"]["email"].value
var atpos=x.indexOf("@");
var dotpos=x.lastIndexOf(".");
if (atpos<1 || dotpos<atpos+2 || dotpos+2>=x.length)
{
alert("Not a valid e-mail address");
return false;
}
}

The function above could be called when a form is submitted:

Example

<form name="myForm" action="demo_form.asp" onsubmit="return validateForm();" method="post">
Email: <input type="text" name="email">
<input type="submit" value="Submit">
</form>

Javascript Cookies

A cookie is often used to identify a user.

What is a Cookie?

A cookie is a variable that is stored on the visitor's computer. Each time the same computer requests a page with a browser, it will send the cookie too. With JavaScript, you can both create and retrieve cookie values.
Examples of cookies:

Name cookie - The first time a visitor arrives to your web page, he or she must fill in her/his name. The name is then stored in a cookie. Next time the visitor arrives at your page, he or she could get a welcome message like "Welcome John Doe!" The name is retrieved from the stored cookie
Password cookie - The first time a visitor arrives to your web page, he or she must fill in a password. The password is then stored in a cookie. Next time the visitor arrives at your page, the password is retrieved from the cookie
Date cookie - The first time a visitor arrives to your web page, the current date is stored in a cookie. Next time the visitor arrives at your page, he or she could get a message like "Your last visit was on Tuesday August 11, 2005!" The date is retrieved from the stored cookie

Create and Store a Cookie

In this example we will create a cookie that stores the name of a visitor. The first time a visitor arrives to the web page, he or she will be asked to fill in her/his name. The name is then stored in a cookie. The next time the visitor arrives at the same page, he or she will get welcome message.
First, we create a function that stores the name of the visitor in a cookie variable:

function setCookie(c_name,value,exdays)
{
var exdate=new Date();
exdate.setDate(exdate.getDate() + exdays);
var c_value=escape(value) + ((exdays==null) ? "" : "; expires="+exdate.toUTCString());
document.cookie=c_name + "=" + c_value;
}

The parameters of the function above hold the name of the cookie, the value of the cookie, and the number of days until the cookie expires.
In the function above we first convert the number of days to a valid date, then we add the number of days until the cookie should expire. After that we store the cookie name, cookie value and the expiration date in the document.cookie object.
Then, we create another function that returns a specified cookie:

The function above makes an array to retrieve cookie names and values, then it checks if the specified cookie exists, and returns the cookie value.
Last, we create the function that displays a welcome message if the cookie is set, and if the cookie is not set it will display a prompt box, asking for the name of the user, and stores the username cookie for 365 days, by calling the setCookie function:

All together now:

Example

<html>
<head>
<script type="text/javascript">
function getCookie(c_name)
{
var i,x,y,ARRcookies=document.cookie.split(";");
for (i=0;i<ARRcookies.length;i++)
{
x=ARRcookies[i].substr(0,ARRcookies[i].indexOf("="));
y=ARRcookies[i].substr(ARRcookies[i].indexOf("=")+1);
x=x.replace(/^\s+|\s+$/g,"");
if (x==c_name)
    {
    return unescape(y);
    }
}
}

function setCookie(c_name,value,exdays)
{
var exdate=new Date();
exdate.setDate(exdate.getDate() + exdays);
var c_value=escape(value) + ((exdays==null) ? "" : "; expires="+exdate.toUTCString());
document.cookie=c_name + "=" + c_value;
}

function checkCookie()
{
var username=getCookie("username");
if (username!=null && username!="")
{
alert("Welcome again " + username);
}
else
{
username=prompt("Please enter your name:","");
if (username!=null && username!="")
    {
    setCookie("username",username,365);
    }
}
}
</script>
</head>
<body onload="checkCookie()">
</body>
</html>

Browser Detection

The Navigator object contains information about the visitor's browser.

Browser Detection

Almost everything in this tutorial works on all JavaScript-enabled browsers. However, there are some things that just don't work on certain browsers - especially on older browsers.
Sometimes it can be useful to detect the visitor's browser, and then serve the appropriate information.
The Navigator object contains information about the visitor's browser name, version, and more.
Note

Note: There is no public standard that applies to the navigator object, but all major browsers support it.

The Navigator Object

The Navigator object contains all information about the visitor's browser:

Example

<div id="example"></div>

<script type="text/javascript">

txt = "<p>Browser CodeName: " + navigator.appCodeName + "</p>";
txt+= "<p>Browser Name: " + navigator.appName + "</p>";
txt+= "<p>Browser Version: " + navigator.appVersion + "</p>";
txt+= "<p>Cookies Enabled: " + navigator.cookieEnabled + "</p>";
txt+= "<p>Platform: " + navigator.platform + "</p>";
txt+= "<p>User-agent header: " + navigator.userAgent + "</p>";

document.getElementById("example").innerHTML=txt;

</script>