Showing posts with label Tutorials. Show all posts
Showing posts with label Tutorials. Show all posts

JavaScript Special Characters
4:28 AM - Posted by Vipin C - 0 comments
In JavaScript you can add special characters to a text string by using the backslash sign.

Insert Special Characters

The backslash (\) is used to insert apostrophes, new lines, quotes, and other special characters into a text string.
Look at the following JavaScript code:

var txt="We are the so-called "Vikings" from the north.";
document.write(txt);		   

In JavaScript, a string is started and stopped with either single or double quotes. This means that the string above will be chopped to: We are the so-called
To solve this problem, you must place a backslash (\) before each double quote in "Viking". This turns each double quote into a string literal:

var txt="We are the so-called \"Vikings\" from the north.";
document.write(txt);

JavaScript will now output the proper text string: We are the so-called "Vikings" from the north.
Here is another example:

document.write ("You \& I are singing!");

The example above will produce the following output:
You & I are singing!
The table below lists other special characters that can be added to a text string with the backslash sign:
Code Outputs
\' single quote
\" double quote
\& ampersand
\\ backslash
\n new line
\r carriage return
\t tab
\b backspace
\f form feed


JavaScript Throw Statement
4:26 AM - Posted by Vipin C - 0 comments
The throw statement allows you to create an exception.


The Throw Statement

The throw statement allows you to create an exception. If you use this statement together with the try...catch statement, you can control program flow and generate accurate error messages.

Syntax

throw(exception)
The exception can be a string, integer, Boolean or an object.
Note that throw is written in lowercase letters. Using uppercase letters will generate a JavaScript error!

Example

The example below determines the value of a variable called x. If the value of x is higher than 10, lower than 0, or not a number, we are going to throw an error. The error is then caught by the catch argument and the proper error message is displayed:


<html>
<body>
<script type="text/javascript">
var x=prompt("Enter a number between 0 and 10:","");
try
  {
  if(x>10)
    {
    throw "Err1";
    }
  else if(x<0)
    {
    throw "Err2";
    }
  else if(isNaN(x))
    {
    throw "Err3";
    }
  }
catch(er)
  {
  if(er=="Err1")
    {
    alert("Error! The value is too high");
    }
  if(er=="Err2")
    {
    alert("Error! The value is too low");
    }
  if(er=="Err3")
    {
    alert("Error! The value is not a number");
    }
  }
</script>
</body>
</html>




Javascript : The try...catch Statement
4:23 AM - Posted by Vipin C - 0 comments
The try...catch statement allows you to test a block of code for errors. The try block contains the code to be run, and the catch block contains the code to be executed if an error occurs.

Syntax

try
  {
  //Run some code here
  }
catch(err)
  {
  //Handle errors here
  }
Note that try...catch is written in lowercase letters. Using uppercase letters will generate a JavaScript error!


Example

<html>
<head>
<script type="text/javascript">
var txt=""
function message()
{
try
  {
  adddlert("Welcome guest!");
  }
catch(err)
  {
  txt="There was an error on this page.\n\n";
  txt+="Error description: " + err.description + "\n\n";
  txt+="Click OK to continue.\n\n";
  alert(txt);
  }
}
</script>
</head>


<body>
<input type="button" value="View message" onclick="message()" />
</body>


</html>


JavaScript - Catching Errors
4:21 AM - Posted by Vipin C - 0 comments
When browsing Web pages on the internet, we all have seen a JavaScript alert box telling us there is a runtime error and asking "Do you wish to debug?". Error message like this may be useful for developers but not for users. When users see errors, they often leave the Web page.

The try...catch statement allows you to test a block of code for errors.


Javascript : onMouseOver and onMouseOut Events
4:20 AM - Posted by Vipin C - 0 comments

onMouseOver and onMouseOut are often used to create "animated" buttons.
Below is an example of an onMouseOver event. An alert box appears when an onMouseOver event is detected:



<a href="http://www.w3schools.com" onmouseover="alert('An onMouseOver event');return false"><img src="w3s.gif" alt="W3Schools" /></a>


Javascript : onSubmit Event
4:19 AM - Posted by Vipin C - 0 comments

The onSubmit event is used to validate ALL form fields before submitting it.
Below is an example of how to use the onSubmit event. The checkForm() function will be called when the user clicks the submit button in the form. If the field values are not accepted, the submit should be cancelled. The function checkForm() returns either true or false. If it returns true the form will be submitted, otherwise the submit will be cancelled:


<form method="post" action="xxx.htm" onsubmit="return checkForm()">


Javascript : onFocus, onBlur and onChange events
4:17 AM - Posted by Vipin C - 0 comments

The onFocus, onBlur and onChange events are often used in combination with validation of form fields.
Below is an example of how to use the onChange event. The checkEmail() function will be called whenever the user changes the content of the field:
<input type="text" size="30" id="email" onchange="checkEmail()">


Javascript onLoad and onUnload Events
4:16 AM - Posted by Vipin C - 0 comments
The onLoad and onUnload events are triggered when the user enters or leaves the page.
The onLoad event is often used to check the visitor's browser type and browser version, and load the proper version of the web page based on the information.
Both the onLoad and onUnload events are also often used to deal with cookies that should be set when a user enters or leaves a page. For example, you could have a popup asking for the user's name upon his first arrival to your page. The name is then stored in a cookie. Next time the visitor arrives at your page, you could have another popup saying something like: "Welcome John Doe!".


JavaScript Events
3:28 AM - Posted by Vipin C - 0 comments
Events are actions that can be detected by JavaScript.

Events

By using JavaScript, we have the ability to create dynamic web pages. Events are actions that can be detected by JavaScript.
Every element on a web page has certain events which can trigger a JavaScript. For example, we can use the onClick event of a button element to indicate that a function will run when a user clicks on the button. We define the events in the HTML tags.
Examples of events:
  • A mouse click
  • A web page or an image loading
  • Mousing over a hot spot on the web page
  • Selecting an input field in an HTML form
  • Submitting an HTML form
  • A keystroke
Note: Events are normally used in combination with functions, and the function will not be executed before the event occurs!


JavaScript For...In Statement
3:25 AM - Posted by Vipin C - 0 comments

The for...in statement loops through the elements of an array or through the properties of an object.

Syntax

for (variable in object)
  {
  code to be executed
  }
Note: The code in the body of the for...in loop is executed once for each element/property.
Note: The variable argument can be a named variable, an array element, or a property of an object.

Example

Use the for...in statement to loop through an array:

Example

<html>
<body>

<script type="text/javascript">
var x;
var mycars = new Array();
mycars[0] = "Saab";
mycars[1] = "Volvo";
mycars[2] = "BMW";

for (x in mycars)
  {
  document.write(mycars[x] + "<br />");
  }
</script>

</body>
</html>


Javascript continue Statement
3:23 AM - Posted by Vipin C - 0 comments
The continue statement will break the current loop and continue with the next value

Example

.
<html>
<body>
<script type="text/javascript">
var i=0
for (i=0;i<=10;i++)
  {
  if (i==3)
    {
    continue;
    }
  document.write("The number is " + i);
  document.write("<br />");
  }
</script>
</body>
</html>


Javascript break Statement
3:22 AM - Posted by Vipin C - 0 comments
The break statement will break the loop and continue executing the code that follows after the loop (if any).

Example



<html>
<body>
<script type="text/javascript">
var i=0;
for (i=0;i<=10;i++)
  {
  if (i==3)
    {
    break;
    }
  document.write("The number is " + i);
  document.write("<br />");
  }
</script>
</body>
</html>


The do...while Loop
3:31 AM - Posted by Vipin C - 0 comments
The do...while loop is a variant of the while loop. This loop will execute the block of code ONCE, and then it will repeat the loop as long as the specified condition is true.

Syntax

do
  {
  code to be executed
  }
while (var<=endvalue);

Example

The example below uses a do...while loop. The do...while loop will always be executed at least once, even if the condition is false, because the statements are executed before the condition is tested:

<html>
<body>
<script type="text/javascript">
var i=0;
do
  {
  document.write("The number is " + i);
  document.write("<br />");
  i++;
  }
while (i<=5);
</script>
</body>
</html>


The while Loop
3:30 AM - Posted by Vipin C - 0 comments
The while loop loops through a block of code while a specified condition is true.

Syntax

while (var<=endvalue)
  {
  code to be executed
  }
Note: The <= could be any comparing statement.

Example

The example below defines a loop that starts with i=0. The loop will continue to run as long as ii will increase by 1 each time the loop runs: is less than, or equal to 5.

<html>
<body>
<script type="text/javascript">
var i=0;
while (i<=5)
  {
  document.write("The number is " + i);
  document.write("<br />");
  i++;
  }
</script>
</body>
</html>



The for Loop
3:28 AM - Posted by Vipin C - 0 comments
The for loop is used when you know in advance how many times the script should run.

Syntax

for (var=startvalue;var<=endvalue;var=var+increment)
{
code to be executed
}

Example

The example below defines a loop that starts with i=0. The loop will continue to run as long as ii will increase by 1 each time the loop runs. is less than, or equal to 5.
Note: The increment parameter could also be negative, and the <= could be any comparing statement.

<html>
<body>
<script type="text/javascript">
var i=0;
for (i=0;i<=5;i++)
{
document.write("The number is " + i);
document.write("<br />");
}
</script>
</body>
</html>



JavaScript Loops
3:26 AM - Posted by Vipin C - 0 comments
Often when you write code, you want the same block of code to run over and over again in a row. Instead of adding several almost equal lines in a script we can use loops to perform a task like this.
In JavaScript, there are two different kind of loops:
  • for - loops through a block of code a specified number of times
  • while - loops through a block of code while a specified condition is true


The Lifetime of JavaScript Variables
3:25 AM - Posted by Vipin C - 0 comments
If you declare a variable within a function, the variable can only be accessed within that function. When you exit the function, the variable is destroyed. These variables are called local variables. You can have local variables with the same name in different functions, because each is recognized only by the function in which it is declared.
If you declare a variable outside a function, all the functions on your page can access it. The lifetime of these variables starts when they are declared, and ends when the page is closed.


The return Statement
3:24 AM - Posted by Vipin C - 0 comments
The return statement is used to specify the value that is returned from the function.
So, functions that are going to return a value must use the return statement.
The example below returns the product of two numbers (a and b):

Example

<html>
<head>
<script type="text/javascript">
function product(a,b)
{
return a*b;
}
</script>
</head>

<body>
<script type="text/javascript">
document.write(product(4,3));
</script>

</body>
</html>


How to Define a Function
3:21 AM - Posted by Vipin C - 0 comments

Syntax

function functionname(var1,var2,...,varX)
{
some code
}
The parameters var1, var2, etc. are variables or values passed into the function. The { and the } defines the start and end of the function.
Note: A function with no parameters must include the parentheses () after the function name.
Note: Do not forget about the importance of capitals in JavaScript! The word function must be written in lowercase letters, otherwise a JavaScript error occurs! Also note that you must call a function with the exact same capitals as in the function name.


JavaScript Functions
3:19 AM - Posted by Vipin C - 0 comments
To keep the browser from executing a script when the page loads, you can put your script into a function.
A function contains code that will be executed by an event or by a call to the function.
You may call a function from anywhere within a page (or even from other pages if the function is embedded in an external .js file).
Functions can be defined both in the and in the section of a document. However, to assure that a function is read/loaded by the browser before it is called, it could be wise to put functions in the section.