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debrider,
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free premium link generator,
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linkgen.us,
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Libellés :
JavaScript and Cookies
JavaScript and Cookies
What are Cookies ?
Web Browser and Server use HTTP protocol to communicate and HTTP is a stateless protocol. But for a commercial website it is required to maintain session information among different pages. For example one user registration ends after completing many pages. But how to maintain user's session information across all the web pages.
In many situations, using cookies is the most efficient method of remembering and tracking preferences, purchases, commissions, and other information required for better visitor experience or site statistics.
How It Works ?
Your server sends some data to the visitor's browser in the form of a cookie. The browser may accept the cookie. If it does, it is stored as a plain text record on the visitor's hard drive. Now, when the visitor arrives at another page on your site, the browser sends the same cookie to the server for retrieval. Once retrieved, your server knows/remembers what was stored earlier.
Cookies are a plain text data record of 5 variable-length fields:
- Expires : The date the cookie will expire. If this is blank, the cookie will expire when the visitor quits the browser.
- Domain : The domain name of your site.
- Path : The path to the directory or web page that set the cookie. This may be blank if you want to retrieve the cookie from any directory or page.
- Secure : If this field contains the word "secure" then the cookie may only be retrieved with a secure server. If this field is blank, no such restriction exists.
- Name=Value : Cookies are set and retrieved in the form of key and value pairs.
Cookies were originally designed for CGI programming and cookies' data is automatically transmitted between the web browser and web server, so CGI scripts on the server can read and write cookie values that are stored on the client.
JavaScript can also manipulate cookies using the cookie property of the Document object. JavaScript can read, create, modify, and delete the cookie or cookies that apply to the current web page.
Storing Cookies:
The simplest way to create a cookie is to assign a string value to the document.cookie object, which looks like this:
Syntax:
document.cookie = "key1=value1;key2=value2;expires=date"; |
Here expires attribute is option. If you provide this attribute with a valid date or time then cookie will expire at the given date or time and after that cookies' value will not be accessible.
Note: Cookie values may not include semicolons, commas, or whitespace. For this reason, you may want to use the JavaScript escape() function to encode the value before storing it in the cookie. If you do this, you will also have to use the corresponding unescape() function when you read the cookie value.
Example:
Following is the example to set a customer name in input cookie.
<html>
<head>
<script type="text/javascript">
<!--
function WriteCookie()
{
if( document.myform.customer.value == "" ){
alert("Enter some value!");
return;
}
cookievalue= escape(document.myform.customer.value) + ";";
document.cookie="name=" + cookievalue;
alert("Setting Cookies : " + "name=" + cookievalue );
}
//-->
</script>
</head>
<body>
<form name="myform" action="">
Enter name: <input type="text" name="customer"/>
<input type="button" value="Set Cookie" onclick="WriteCookie();"/>
</form>
</body>
</html>
|
This will produce following result. Now enter something in the text box and press the button "Set Cookie" to set the cookies.
Now your machine has a cookie called name. You can set multiple cookies using multiplekey=value pairs separated by comma.
You will learn how to read this cookie in next section.
Reading Cookies:
Reading a cookie is just as simple as writing one, because the value of the document.cookieobject is the cookie. So you can use this string whenever you want to access the cookie.
The document.cookie string will keep a list of name=value pairs separated by semicolons, wherename is the name of a cookie and value is its string value.
You can use strings' split() function to break the string into key and values as follows:
Example:
Following is the example to get the cookies set in previous section.
<html>
<head>
<script type="text/javascript">
<!--
function ReadCookie()
{
var allcookies = document.cookie;
alert("All Cookies : " + allcookies );
// Get all the cookies pairs in an array
cookiearray = allcookies.split(';');
// Now take key value pair out of this array
for(var i=0; i<cookiearray.length; i++){
name = cookiearray[i].split('=')[0];
value = cookiearray[i].split('=')[1];
alert("Key is : " + name + " and Value is : " + value);
}
}
//-->
</script>
</head>
<body>
<form name="myform" action="">
<input type="button" value="Get Cookie" onclick="ReadCookie()"/>
</form>
</body>
</html>
|
Note: Here length is a method of Array class which returns the length of an array. We will discuss Arrays in a separate chapter. By that time please try to digest it.
This will produce following result. Now press the button "Get Cookie" to see the cookies which you have set in previous section.
Note: There may be some other cookies already set on your machine. So above code will show you all the cookies set at your machine.
Setting the Cookies Expiration Date:
You can extend the life of a cookie beyond the current browser session by setting an expiration date and saving the expiration date within the cookie. This can be done by setting the expiresattribute to a date and time.
Example:
The following example illustrates how to set cookie expiration date after 1 Month :
<html>
<head>
<script type="text/javascript">
<!--
function WriteCookie()
{
var now = new Date();
now.setMonth( now.getMonth() + 1 );
cookievalue = escape(document.myform.customer.value) + ";"
document.cookie="name=" + cookievalue;
document.cookie = "expires=" + now.toUTCString() + ";"
alert("Setting Cookies : " + "name=" + cookievalue );
}
//-->
</script>
</head>
<body>
<form name="formname" action="">
Enter name: <input type="text" name="customer"/>
<input type="button" value="Set Cookie" onclick="WriteCookie()"/>
</form>
</body>
</html>
|
Deleting a Cookie:
Sometimes you will want to delete a cookie so that subsequent attempts to read the cookie return nothing. To do this, you just need to set the expiration date to a time in the past.
Example:
The following example illustrates how to delete cookie by setting expiration date one Month in past :
<html>
<head>
<script type="text/javascript">
<!--
function WriteCookie()
{
var now = new Date();
now.setMonth( now.getMonth() - 1 );
cookievalue = escape(document.myform.customer.value) + ";"
document.cookie="name=" + cookievalue;
document.cookie = "expires=" + now.toUTCString() + ";"
alert("Setting Cookies : " + "name=" + cookievalue );
}
//-->
</script>
</head>
<body>
<form name="formname" action="">
Enter name: <input type="text" name="customer"/>
<input type="button" value="Set Cookie" onclick="WriteCookie()"/>
</form>
</body>
</html>
|
Note: Instead of setting date, you can see new time using setTime() function.
Libellés :
JavaScript Events
JavaScript Events
What is an Event ?
JavaScript's interaction with HTML is handled through events that occur when the user or browser manipulates a page.
When the page loads, that is an event. When the user clicks a button, that click, too, is an event. Another example of events are like pressing any key, closing window, resizing window etc.
Developers can use these events to execute JavaScript coded responses, which cause buttons to close windows, messages to be displayed to users, data to be validated, and virtually any other type of response imaginable to occur.
Events are a part of the Document Object Model (DOM) Level 3 and every HTML element have a certain set of events which can trigger JavaScript Code.
Here we will see few examples to understand a relation between Event and JavaScript:
onclick Event Type:
This is the most frequently used event type which occurs when a user clicks mouse left button. You can put your validation, warning etc against this event type.
Example:
<html>
<head>
<script type="text/javascript">
<!--
function sayHello() {
alert("Hello World")
}
//-->
</script>
</head>
<body>
<input type="button" onclick="sayHello()" value="Say Hello" />
</body>
</html>
|
This will produce following result and when you click Hello button then onclick event will occur which will trigger sayHello() function.
onsubmit event type:
Another most important event type is onsubmit. This event occurs when you try to submit a form. So you can put your form validation against this event type.
Here is simple example showing its usage. Here we are calling a validate() function before submitting a form data to the webserver. If validate() function returns true the form will be submitted otherwise it will not submit the data.
Example:
<html>
<head>
<script type="text/javascript">
<!--
function validation() {
all validation goes here
.........
return either true or false
}
//-->
</script>
</head>
<body>
<form method="POST" action="t.cgi" onsubmit="return validate()">
.......
<input type="submit" value="Submit" />
</form>
</body>
</html>
|
onmouseover and onmouseout:
These two event types will help you to create nice effects with images or even with text as well. The onmouseover event occurs when you bring your mouse over any element and theonmouseout occurs when you take your mouse out from that element.
Example:
Following example shows how a division reacts when we bring our mouse in that division:
<html>
<head>
<script type="text/javascript">
<!--
function over() {
alert("Mouse Over");
}
function out() {
alert("Mouse Out");
}
//-->
</script>
</head>
<body>
<div onmouseover="over()" onmouseout="out()">
<h2> This is inside the division </h2>
</div>
</body>
</html>
|
You can change different images using these two event types or you can create help baloon to help your users.
HTML 4 Standard Events
The standard HTML 4 events are listed here for your reference. Here script indicates a Javascript function to be executed agains that event.
| Event | Value | Description |
|---|---|---|
| onchange | script | Script runs when the element changes |
| onsubmit | script | Script runs when the form is submitted |
| onreset | script | Script runs when the form is reset |
| onselect | script | Script runs when the element is selected |
| onblur | script | Script runs when the element loses focus |
| onfocus | script | Script runs when the element gets focus |
| onkeydown | script | Script runs when key is pressed |
| onkeypress | script | Script runs when key is pressed and released |
| onkeyup | script | Script runs when key is released |
| onclick | script | Script runs when a mouse click |
| ondblclick | script | Script runs when a mouse double-click |
| onmousedown | script | Script runs when mouse button is pressed |
| onmousemove | script | Script runs when mouse pointer moves |
| onmouseout | script | Script runs when mouse pointer moves out of an element |
| onmouseover | script | Script runs when mouse pointer moves over an element |
| onmouseup | script | Script runs when mouse button is released |
Libellés :
JavaScript Functions
JavaScript Functions
A function is a group of reusable code which can be called anywhere in your programme. This eliminates the need of writing same code again and again. This will help programmers to write modular code. You can divide your big programme in a number of small and manageable functions.
Like any other advance programming language, JavaScript also supports all the features necessary to write modular code using functions.
You must have seen functions like alert() and write() in previous chapters. We are using these function again and again but they have been written in core JavaScript only once.
JavaScript allows us to write our own functions as well. This section will explain you how to write your own functions in JavaScript.
Function Definition:
Before we use a function we need to define that function. The most common way to define a function in JavaScript is by using the function keyword, followed by a unique function name, a list of parameters (that might be empty), and a statement block surrounded by curly braces. The basic syntax is shown here:
<script type="text/javascript">
<!--
function functionname(parameter-list)
{
statements
}
//-->
</script>
|
Example:
A simple function that takes no parameters called sayHello is defined here:
<script type="text/javascript">
<!--
function sayHello()
{
alert("Hello there");
}
//-->
</script>
|
Calling a Function:
To invoke a function somewhere later in the script, you would simple need to write the name of that function as follows:
<script type="text/javascript"> <!-- sayHello(); //--> </script> |
Function Parameters:
Till now we have seen function without a parameters. But there is a facility to pass different parameters while calling a function. These passed parameters can be captured inside the function and any manipulation can be done over those parameters.
A function can take multiple parameters separated by comma.
Example:
Let us do a bit modification in our sayHello function. This time it will take two parameters:
<script type="text/javascript">
<!--
function sayHello(name, age)
{
alert( name + " is " + age + " years old.");
}
//-->
</script>
|
Note: We are using + operator to concatenate string and number all together. JavaScript does not mind in adding numbers into strings.
Now we can call this function as follows:
<script type="text/javascript">
<!--
sayHello('Zara', 7 );
//-->
</script>
|
The return Statement:
A JavaScript function can have an optional return statement. This is required if you want to return a value from a function. This statement should be the last statement in a function.
For example you can pass two numbers in a function and then you can expect from the function to return their multiplication in your calling program.
Example:
This function takes two parameters and concatenates them and return resultant in the calling program:
<script type="text/javascript">
<!--
function concatenate(first, last)
{
var full;
full = first + last;
return full;
}
//-->
</script>
|
Now we can call this function as follows:
<script type="text/javascript">
<!--
var result;
result = concatenate('Zara', 'Ali');
alert(result );
//-->
</script>
|
Libellés :
JavaScript Loop Control
JavaScript Loop Control
JavaScript provides you full control to handle your loops and switch statement. There may be a situation when you need to come out of a loop without reaching at its bottom. There may also be a situation when you want to skip a part of your code block and want to start next iteration of the look.
To handle all such situations, JavaScript provides break and continue statements. These statements are used to immediately come out of any loop or to start the next iteration of any loop respectively.
The break Statement:
The break statement, which was briefly introduced with the switch statement, is used to exit a loop early, breaking out of the enclosing curly braces.
Example:
This example illustrates the use of a break statement with a while loop. Notice how the loop breaks out early once x reaches 5 and reaches to document.write(..) statement just below to closing curly brace:
<script type="text/javascript">
<!--
var x = 1;
document.write("Entering the loop<br /> ");
while (x < 20)
{
if (x == 5){
break; // breaks out of loop completely
}
x = x + 1;
document.write( x + "<br />");
}
document.write("Exiting the loop!<br /> ");
//-->
</script>
|
This will produce following result:
Entering the loop 2 3 4 5 Exiting the loop! |
We already have seen the usage of break statement inside a switch statement.
The continue Statement:
The continue statement tells the interpreter to immediately start the next iteration of the loop and skip remaining code block.
When a continue statement is encountered, program flow will move to the loop check expression immediately and if condition remain true then it start next iteration otherwise control comes out of the loop.
Example:
This example illustrates the use of a continue statement with a while loop. Notice how thecontinue statement is used to skip printing when the index held in variable x reaches 5:
<script type="text/javascript">
<!--
var x = 1;
document.write("Entering the loop<br /> ");
while (x < 10)
{
x = x + 1;
if (x == 5){
continue; // skill rest of the loop body
}
document.write( x + "<br />");
}
document.write("Exiting the loop!<br /> ");
//-->
</script>
|
This will produce following result:
Entering the loop 2 3 4 6 7 8 9 10 Exiting the loop! |
Using Labels to Control the Flow:
Starting from JavaScript 1.2, a label can be used with break and continue to control the flow more precisely.
A label is simply an identifier followed by a colon that is applied to a statement or block of code. We will see two different examples to understand label with break and continue.
Note: Line breaks are not allowed between the continue or break statement and its label name. Also, there should not be any other statement in between a label name and associated loop.
Example 1:
<script type="text/javascript">
<!--
document.write("Entering the loop!<br /> ");
outerloop: // This is the label name
for (var i = 0; i < 5; i++)
{
document.write("Outerloop: " + i + "<br />");
innerloop:
for (var j = 0; j < 5; j++)
{
if (j > 3 ) break ; // Quit the innermost loop
if (i == 2) break innerloop; // Do the same thing
if (i == 4) break outerloop; // Quit the outer loop
document.write("Innerloop: " + j + " <br />");
}
}
document.write("Exiting the loop!<br /> ");
//-->
</script>
|
This will produce following result:
Entering the loop! Outerloop: 0 Innerloop: 0 Innerloop: 1 Innerloop: 2 Innerloop: 3 Outerloop: 1 Innerloop: 0 Innerloop: 1 Innerloop: 2 Innerloop: 3 Outerloop: 2 Outerloop: 3 Innerloop: 0 Innerloop: 1 Innerloop: 2 Innerloop: 3 Outerloop: 4 Exiting the loop! |
Example 2:
<script type="text/javascript">
<!--
document.write("Entering the loop!<br /> ");
outerloop: // This is the label name
for (var i = 0; i < 3; i++)
{
document.write("Outerloop: " + i + "<br />");
for (var j = 0; j < 5; j++)
{
if (j == 3){
continue outerloop;
}
document.write("Innerloop: " + j + "<br />");
}
}
document.write("Exiting the loop!<br /> ");
//-->
</script>
|
This will produce following result:
Entering the loop! Outerloop: 0 Innerloop: 0 Innerloop: 1 Innerloop: 2 Outerloop: 1 Innerloop: 0 Innerloop: 1 Innerloop: 2 Outerloop: 2 Innerloop: 0 Innerloop: 1 Innerloop: 2 Exiting the loop! |
Libellés :
JavaScript for...in loop
JavaScript for...in loop
There is one more loop supported by JavaScript. It is called for...in loop. This loop is used to loop through an object's properties.
Because we have not discussed Objects yet, so you may not feel comfortable with this loop. But once you will have understanding on JavaScript objects then you will find this loop very useful.
Syntax:
for (variablename in object){
statement or block to execute
}
|
In each iteration one property from object is assigned to variablename and this loop continues till all the properties of the object are exhausted.
Example:
Here is the following example that prints out the properties of a Web browser's Navigatorobject:
<script type="text/javascript">
<!--
var aProperty;
document.write("Navigator Object Properties<br /> ");
for (aProperty in navigator)
{
document.write(aProperty);
document.write("<br />");
}
document.write("Exiting from the loop!");
//-->
</script>
|
This will produce following result:
Navigator Object Properties appCodeName appName appMinorVersion cpuClass platform plugins opsProfile userProfile systemLanguage userLanguage appVersion userAgent onLine cookieEnabled mimeTypes Exiting from the loop! |
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