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HTML and Javascript Tutorial

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HTML Basic Document

<html>

<head>

<title>Document name goes here</title>

</head>

<body>

Visible text goes here

</body>

</html>

Heading Elements

<h1>Largest Heading</h1>

<h2> . . . </h2>

<h3> . . . </h3>

<h4> . . . </h4>

<h5> . . . </h5>

<h6>Smallest Heading</h6>

Text Elements

<p>This is a paragraph</p>

<br> (line break)

<hr> (horizontal rule)

<pre>This text is preformatted</pre>

Logical Styles

<em>This text is emphasized</em>

<strong>This text is strong</strong>

<code>This is some computer code</code>

Physical Styles

<b>This text is bold</b>

<i>This text is italic</i>

Links, Anchors, and Image Elements

<a href="http://www.example.com/">This is a Link</a>

<a href="http://www.example.com/"><img src="URL" alt="Alternate Text"></a>

<a href="mailto:[email protected]">Send e-mail</a>

A named anchor:

<a name="tips">Useful Tips Section</a>

<a href="#tips">Jump to the Useful Tips Section</a>

Unordered list

<ul>

<li>First item</li>

<li>Next item</li>

</ul>

Ordered list

<ol>

<li>First item</li>

<li>Next item</li>

</ol>

Definition list

<dl>

<dt>First term</dt>

<dd>Definition</dd>

<dt>Next term</dt>

<dd>Definition</dd>

</dl>

Tables

<table border="1">

<tr>

<th>someheader</th>

<th>someheader</th>

</tr>

<tr>

<td>sometext</td>

<td>sometext</td>

</tr>

</table>

Frames

<frameset cols="25%,75%">

<frame src="page1.htm">

<frame src="page2.htm">

</frameset>

Forms

<form action="http://www.example.com/test.asp" method="post/get">

<input type="text" name="lastname" value="Nixon" size="30" maxlength="50">

<input type="password">

<input type="checkbox" checked="checked">

<input type="radio" checked="checked">

<input type="submit">

<input type="reset">

<input type="hidden">

<select>

<option>Apples

<option selected>Bananas

<option>Cherries

</select>

<textarea name="Comment" rows="60" cols="20"></textarea>

</form>

Entities

&lt; is the same as <

&gt; is the same as >

&#169; is the same as ©

Other Elements

<!-- This is a comment -->

<blockquote>

Text quoted from some source.

</blockquote>

<address>

Address 1<br>

Address 2<br>

City<br>

</address>

Commonly Used Character Entities

Note Entity names are case sensitive!

Result

Description

Entity Name

Entity Number

 

non-breaking space

&nbsp;

&#160;

<

less than

&lt;

&#60;

>

greater than

&gt;

&#62;

&

ampersand

&amp;

&#38;

¢

cent

&cent;

&#162;

£

pound

&pound;

&#163;

¥

yen

&yen;

&#165;

euro

&euro;

&#8364;

§

section

&sect;

&#167;

©

copyright

&copy;

&#169;

®

registered trademark

&reg;

&#174;

The Meta Element

As we explained in the previous chapter, the head element contains general information (meta-information) about a document.

HTML also includes a meta element that goes inside the head element. The purpose of the meta element is to provide meta-information about the document.

Most often the meta element is used to provide information that is relevant to browsers or search engines like describing the content of your document.

Keywords for Search Engines

Some search engines on the WWW will use the name and content attributes of the meta tag to index your pages.

This meta element defines a description of your page:

<meta name="description" content="Free Web tutorials on HTML, CSS, XML, and XHTML">

This meta element defines keywords for your page:

<meta name="keywords" content="HTML, DHTML, CSS, XML, XHTML, JavaScript, VBScript">

The intention of the name and content attributes is to describe the content of a page.

However, since too many webmasters have used meta tags for spamming, like repeating keywords to give pages a higher ranking, some search engines have stopped using them entirely.

Uniform Resource Locators

Something called a Uniform Resource Locator (URL) is used to address a document (or other data) on the World Wide Web. A full Web address like this: http://www.w3schools.com/html/lastpage.htm follows these syntax rules:

scheme://host.domain:port/path/filename

The scheme is defining the type of Internet service. The most common type is http.

The domain is defining the Internet domain name like w3schools.com.

The host is defining the domain host. If omitted, the default host for http is www.

The :port is defining the port number at the host. The port number is normally omitted. The default port number for http is 80.

The path is defining a path (a sub directory) at the server. If the path is omitted, the resource (the document) must be located at the root directory of the Web site.

The filename is defining the name of a document. The default filename might be default.asp, or index.html or something else depending on the settings of the Web server.

URL Schemes

Some examples of the most common schemes can be found below:

Schemes

Access

file

a file on your local PC

ftp

a file on an FTP server

http

a file on a World Wide Web Server

gopher

a file on a Gopher server

news

a Usenet newsgroup

telnet

a Telnet connection

WAIS

a file on a WAIS server

Accessing a Newsgroup

The following HTML code:

<a href="news:alt.html">HTML Newsgroup</a>

creates a link to a newsgroup like this HTML Newsgroup

Downloading with FTP

The following HTML code:

<a href="ftp://www.w3schools.com/ftp/winzip.exe">Download WinZip</a>

creates a link to download a file like this: Download WinZip.

(The link doesn't work. Don't try it. It is just an example. W3Schools doesn't really have an ftp directory.)

Link to your Mail system

The following HTML code:

<a href="mailto:[email protected]">[email protected]</a>

creates a link to your own mail system like this:

Insert a Script into HTML Page

A script in HTML is defined with the <script> tag. Note that you will have to use the type attribute to specify the scripting language.

<html>

<head>

</head>

<body>

<script type="text/javascript">

document.write("Hello World!")

</script>

</body>

</html>

How to Handle Older Browsers

A browser that does not recognize the <script> tag at all, will display the <script> tag's content as text on the page. To prevent the browser from doing this, you should hide the script in comment tags. An old browser (that does not recognize the <script> tag) will ignore the comment and it will not write the tag's content on the page, while a new browser will understand that the script should be executed, even if it is surrounded by comment tags.

Example

JavaScript:

<script type="text/javascript">

<!--

document.write("Hello World!")

//-->

</script>

VBScript:

<script type="text/vbscript">

<!--

document.write("Hello World!")

'-->

</script>

New to HTML 4.0 is the ability to let HTML events trigger actions in the browser, like starting a JavaScript when a user clicks on an HTML element. Below is a list of attributes that can be inserted into HTML tags to define event actions.

Window Events

Only valid in body and frameset elements.

Attribute

Value

Description

onload

script

Script to be run when a document loads

onunload

script

Script to be run when a document unloads

Only valid in form elements.

Attribute

Value

Description

onchange

script

Script to be run when the element changes

onsubmit

script

Script to be run when the form is submitted

onreset

script

Script to be run when the form is reset

onselect

script

Script to be run when the element is selected

onblur

script

Script to be run when the element loses focus

onfocus

script

Script to be run when the element gets focus

Keyboard Events

Not valid in base, bdo, br, frame, frameset, head, html, iframe, meta, param, script, style, and title elements.

Attribute

Value

Description

onkeydown

script

What to do when key is pressed

onkeypress

script

What to do when key is pressed and released

onkeyup

script

What to do when key is released

Mouse Events

Not valid in base, bdo, br, frame, frameset, head, html, iframe, meta, param, script, style, title elements.

Attribute

Value

Description

onclick

script

What to do on a mouse click

ondblclick

script

What to do on a mouse double-click

onmousedown

script

What to do when mouse button is pressed

onmousemove

script

What to do when mouse pointer moves

onmouseout

script

What to do when mouse pointer moves out of an element

onmouseover

script

What to do when mouse pointer moves over an element

onmouseup

script

What to do when mouse button is released

Your Windows PC as a Web Server

If you want other people to view your pages, you must publish them.

To publish your work, you must save your pages on a web server.

Your own PC can act as a web server if you install IIS or PWS.

IIS or PWS turns your computer into a web server.

Microsoft IIS and PWS are free web server components.

IIS - Internet Information Server

IIS is for Windows system like Windows 2000, XP, and Vista. It is also available for Windows NT.

IIS is easy to install and ideal for developing and testing web applications.

IIS includes Active Server Pages (ASP), a server-side scripting standard that can be used to create dynamic and interactive web applications.

PWS - Personal Web Server

PWS is for older Windows system like Windows 95, 98, and NT.

PWS is easy to install and can be used for developing and testing web applications including ASP.

We don't recommend running PWS for anything else than training. It is outdated and have security issues.

Windows Web Server Versions

Windows Vista Professional comes with IIS 6.

Windows Vista Home Edition does not support PWS or IIS.

Windows XP Professional comes with IIS 5.

Windows XP Home Edition does not support IIS or PWS.

Windows 2000 Professional comes with IIS 4.

Windows NT Professional comes with IIS 3 and also supports IIS 4.

Windows NT Workstation supports PWS and IIS 3.

Windows ME does not support PWS or IIS.

Windows 98 comes with PWS.

Windows 95 supports PWS.

HTML Summary

This tutorial has taught you how to use HTML to create your own web site.

HTML is the universal markup language for the Web. HTML lets you format text, add graphics, create links, input forms, frames and tables, etc., and save it all in a text file that any browser can read and display.

The key to HTML is the tags, which indicates what content is coming up.

XHTML

XHTML reformulates HTML 4.01 in XML.

CSS

CSS is used to control the style and layout of multiple Web pages all at once.

With CSS, all formatting can be removed from the HTML document and stored in a separate file.

CSS gives you total control of the layout, without messing up the document content.

JavaScript Tutorial

What is JavaScript?

JavaScript was designed to add interactivity to HTML pages

JavaScript is a scripting language

A scripting language is a lightweight programming language

JavaScript is usually embedded directly into HTML pages

JavaScript is an interpreted language (means that scripts execute without preliminary compilation)

Everyone can use JavaScript without purchasing a license

What can a JavaScript Do?

JavaScript gives HTML designers a programming tool - HTML authors are normally not programmers, but JavaScript is a scripting language with a very simple syntax! Almost anyone can put small "snippets" of code into their HTML pages

JavaScript can put dynamic text into an HTML page - A JavaScript statement like this: document.write("<h1>" + name + "</h1>") can write a variable text into an HTML page

JavaScript can react to events - A JavaScript can be set to execute when something happens, like when a page has finished loading or when a user clicks on an HTML element

JavaScript can read and write HTML elements - A JavaScript can read and change the content of an HTML element

JavaScript can be used to validate data - A JavaScript can be used to validate form data before it is submitted to a server. This saves the server from extra processing

JavaScript can be used to detect the visitor's browser - A JavaScript can be used to detect the visitor's browser, and - depending on the browser - load another page specifically designed for that browser

JavaScript can be used to create cookies - A JavaScript can be used to store and retrieve information on the visitor's computer

How to Put a JavaScript Into an HTML Page

<html>

<body>

<script type="text/javascript">

document.write("Hello World!");

</script>

</body>

</html>

Where to Put the JavaScript

JavaScripts in a page will be executed immediately while the page loads into the browser. This is not always what we want. Sometimes we want to execute a script when a page loads, other times when a user triggers an event.

Scripts in the head section: Scripts to be executed when they are called, or when an event is triggered, go in the head section. When you place a script in the head section, you will ensure that the script is loaded before anyone uses it.

<html>

<head>

<script type="text/javascript">

....

</script>

</head>

Scripts in the body section: Scripts to be executed when the page loads go in the body section. When you place a script in the body section it generates the content of the page.

<html>

<head>

</head>

<body>

<script type="text/javascript">

....

</script>

</body>

Scripts in both the body and the head section: You can place an unlimited number of scripts in your document, so you can have scripts in both the body and the head section.

<html>

<head>

<script type="text/javascript">

....

</script>

</head>

<body>

<script type="text/javascript">

....

</script>

</body>

Using an External JavaScript

Sometimes you might want to run the same JavaScript on several pages, without having to write the same script on every page.

To simplify this, you can write a JavaScript in an external file. Save the external JavaScript file with a .js file extension.

Note: The external script cannot contain the <script> tag!

To use the external script, point to the .js file in the "src" attribute of the <script> tag:

<html>

<head>

<script type="text/javascript" src="xxx.js"></script>

</head>

<body>

</body>

</html>

JavaScript is Case Sensitive

Unlike HTML, JavaScript is case sensitive - therefore watch your capitalization closely when you write JavaScript statements, create or call variables, objects and functions.

JavaScript Statements

A JavaScript statement is a command to the browser. The purpose of the command is to tell the browser what to do.

This JavaScript statement tells the browser to write "Hello Dolly" to the web page:

document.write("Hello Dolly");

It is normal to add a semicolon at the end of each executable statement. Most people think this is a good programming practice, and most often you will see this in JavaScript examples on the web.

The semicolon is optional (according to the JavaScript standard), and the browser is supposed to interpret the end of the line as the end of the statement. Because of this you will often see examples without the semicolon at the end.

Note: Using semicolons makes it possible to write multiple statements on one line.

JavaScript Code

JavaScript code (or just JavaScript) is a sequence of JavaScript statements.

Each statement is executed by the browser in the sequence they are written.

This example will write a header and two paragraphs to a web page:

<script type="text/javascript">

document.write("<h1>This is a header</h1>");

document.write("<p>This is a paragraph</p>");

document.write("<p>This is another paragraph</p>");

JavaScript Blocks

JavaScript statements can be grouped together in blocks.

Blocks start with a left curly bracket {, and ends with a right curly bracket }.

The purpose of a block is to make the sequence of statements execute together.

This example will write a header and two paragraphs to a web page:

<script type="text/javascript">

{

document.write("<h1>This is a header</h1>");

document.write("<p>This is a paragraph</p>");

document.write("<p>This is another paragraph</p>");

}

</script>

JavaScript comments can be used to make the code more readable.

JavaScript Comments

Comments can be added to explain the JavaScript, or to make it more readable.

Single line comments start with //.

This example uses single line comments to explain the code:

<script type="text/javascript">

// This will write a header:

document.write("<h1>This is a header</h1>");

// This will write two paragraphs:

document.write("<p>This is a paragraph</p>");

document.write("<p>This is another paragraph</p>");

</script>

Using Comments to Prevent Execution

In this example the comment is used to prevent the execution of a single code line:

<script type="text/javascript">

document.write("<h1>This is a header</h1>");

document.write("<p>This is a paragraph</p>");

//document.write("<p>This is another paragraph</p>");

</script>

In this example the comments is used to prevent the execution of multiple code lines:

<script type="text/javascript">

/*

document.write("<h1>This is a header</h1>");

document.write("<p>This is a paragraph</p>");

document.write("<p>This is another paragraph</p>");

*/

</script>

Using Comments at the End of a Line

In this example the comment is placed at the end of a line:

<script type="text/javascript">

document.write("Hello"); // This will write "Hello"

document.write("Dolly"); // This will write "Dolly"

</script>

Variables are "containers" for storing information.

Declaring (Creating) JavaScript Variables

Creating variables in JavaScript is most often referred to as "declaring" variables.

You can declare JavaScript variables with the var statement:

var x;

var carname;

After the declaration shown above, the variables are empty (they have no values yet).

However, you can also assign values to the variables when you declare them:

var x=5;

var carname="Volvo";

After the execution of the statements above, the variable x will hold the value 5, and carname will hold the value Volvo.

Note: When you assign a text value to a variable, use quotes around the value.

Assigning Values to Undeclared JavaScript Variables

If you assign values to variables that have not yet been declared, the variables will automatically be declared.

These statements:

x=5;

carname="Volvo";

have the same effect as:

var x=5;

var carname="Volvo";

Redeclaring JavaScript Variables

If you redeclare a JavaScript variable, it will not lose its original value.

var x=5;

var x;

After the execution of the statements above, the variable x will still have the value of 5. The value of x is not reset (or cleared) when you redeclare it.

Conditional Statements

Very often when you write code, you want to perform different actions for different decisions. You can use conditional statements in your code to do this.

In JavaScript we have the following conditional statements:

if statement - use this statement if you want to execute some code only if a specified condition is true

if...else statement - use this statement if you want to execute some code if the condition is true and another code if the condition is false

if...else if....else statement - use this statement if you want to select one of many blocks of code to be executed

switch statement - use this statement if you want to select one of many blocks of code to be executed

script type="text/javascript">

//If the time is less than 10,

//you will get a "Good morning" greeting.

//Otherwise you will get a "Good day" greeting.

var d = new Date();

var time = d.getHours();

if (time < 10)

{

document.write("Good morning!");

}

else

{

document.write("Good day!");

}

</script>

The JavaScript Switch Statement

You should use the switch statement if you want to select one of many blocks of code to be executed.

Syntax

switch(n)

{

case 1:

execute code block 1

break;

case 2:

execute code block 2

break;

default:

code to be executed if n is

different from case 1 and 2

}

<script type="text/javascript">

//You will receive a different greeting based

//on what day it is. Note that Sunday=0,

//Monday=1, Tuesday=2, etc.

var d=new Date();

theDay=d.getDay();

switch (theDay)

{

case 5:

document.write("Finally Friday");

break;

case 6:

document.write("Super Saturday");

break;

case 0:

document.write("Sleepy Sunday");

break;

default:

document.write("I'm looking forward to this weekend!");

}

</script>

JavaScript Popup Boxes

Alert Box

An alert box is often used if you want to make sure information comes through to the user.

When an alert box pops up, the user will have to click "OK" to proceed.

Syntax:

alert("sometext");

Confirm Box

A confirm box is often used if you want the user to verify or accept something.

When a confirm box pops up, the user will have to click either "OK" or "Cancel" to proceed.

If the user clicks "OK", the box returns true. If the user clicks "Cancel", the box returns false.

Syntax:

confirm("sometext");

Prompt Box

A prompt box is often used if you want the user to input a value before entering a page.

When a prompt box pops up, the user will have to click either "OK" or "Cancel" to proceed after entering an input value.

If the user clicks "OK" the box returns the input value. If the user clicks "Cancel" the box returns null.

Syntax:

prompt("sometext","defaultvalue");

JavaScript Functions

JavaScript Functions

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 that function.

You may call a function from anywhere within the page (or even from other pages if the function is embedded in an external .js file).

Functions can be defined both in the <head> and in the <body> section of a document. However, to assure that the function is read/loaded by the browser before it is called, it could be wise to put it in the <head> section.

How to Define a Function

The syntax for creating a function is:

function functionname(var1,var2,...,varX)

{

some code

}

The return Statement

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.

Example

The function below should return the product of two numbers (a and b):

function prod(a,b)

{

x=a*b;

return x;

}

<html>

<head>

<script type="text/javascript">

function displaymessage()

{

alert("Hello World!");

}

</script>

</head>

<body>

<form>

<input type="button" value="Click me!"

onclick="displaymessage()" >

</form>

</body>

</html>

JavaScript Loops

Very 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 for Loop

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

Explanation: The example below defines a loop that starts with i=0. The loop will continue to run as long as i is less than, or equal to 10. i will increase by 1 each time the loop runs.

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<=10;i++)

{

document.write("The number is " + i);

document.write("<br />");

}

</script>

</body>

</html>

The while loop

The while loop is used when you want the loop to execute and continue executing while the specified condition is true.

while (var<=endvalue)

{

code to be executed

}

Note: The <= could be any comparing statement.

Example

Explanation: The example below defines a loop that starts with i=0. The loop will continue to run as long as i is less than, or equal to 10. i will increase by 1 each time the loop runs.

<html>

<body>

<script type="text/javascript">

var i=0;

while (i<=10)

{

document.write("The number is " + i);

document.write("<br />");

i=i+1;

}

</script>

</body>

</html>

The do...while Loop

The do...while loop is a variant of the while loop. This loop will always execute a block of code ONCE, and then it will repeat the loop as long as the specified condition is true. This loop will always be executed at least once, even if the condition is false, because the code is executed before the condition is tested.

do

{

code to be executed

}

while (var<=endvalue);

Example

<html>

<body>

<script type="text/javascript">

var i=0;

do

{

document.write("The number is " + i);

document.write("<br />");

i=i+1;

}

while (i<0);

</script>

</body>

</html>

JavaScript break and continue Statements

There are two special statements that can be used inside loops: break and continue.

Break

The break command 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>

Continue

The continue command 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 For...In Statement

The for...in statement is used to loop (iterate) through the elements of an array or through the properties of an object.

The code in the body of the for ... in loop is executed once for each element/property.

Syntax

for (variable in object)

{

code to be executed

}

The variable argument can be a named variable, an array element, or a property of an object.

Example

Using for...in to loop through an array:

<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>

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 JavaScript functions. 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 box 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!

onload and onUnload

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!".

onFocus, onBlur and onChange

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()">

onSubmit

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()">

onMouseOver and onMouseOut

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="w3schools.gif" width="100" height="30">

</a>

JavaScript Try...Catch Statement

JavaScript - Catching Errors

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.

This chapter will teach you how to trap and handle JavaScript error messages, so you don't lose your audience.

There are two ways of catching errors in a Web page:

By using the try...catch statement (available in IE5+, Mozilla 1.0, and Netscape 6)

By using the onerror event. This is the old standard solution to catch errors (available since Netscape 3)

Try...Catch Statement

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 1

The example below contains a script that is supposed to display the message "Welcome guest!" when you click on a button. However, there's a typo in the message() function. alert() is misspelled as adddlert(). A JavaScript error occurs:

<html>

<head>

<script type="text/javascript">

function message()

{

adddlert("Welcome guest!");

}

</script>

</head>

<body>

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

</body>

</html>

To take more appropriate action when an error occurs, you can add a try...catch statement.

The example below contains the "Welcome guest!" example rewritten to use the try...catch statement. Since alert() is misspelled, a JavaScript error occurs. However, this time, the catch block catches the error and executes a custom code to handle it. The code displays a custom error message informing the user what happened:

<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>

Example 2

The next example uses a confirm box to display a custom message telling users they can click OK to continue viewing the page or click Cancel to go to the homepage. If the confirm method returns false, the user clicked Cancel, and the code redirects the user. If the confirm method returns true, the code does nothing:

<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+="Click OK to continue viewing this page,\n";

txt+="or Cancel to return to the home page.\n\n";

if(!confirm(txt))

{

document.location.href="http://www.w3schools.com/";

}

}

}

</script>

</head>

<body>

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

</body>

</html>

The onerror Event

The onerror event will be explained soon, but first you will learn how to use the throw statement to create an exception. The throw statement can be used together with the try...catch statement.

JavaScript Throw Statement

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 1

The example below determines the value of a variable called x. If the value of x is higher than 10 or lower than 0 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";

}

catch(er)

{

if(er=="Err1")

alert("Error! The value is too high");

if(er == "Err2")

alert("Error! The value is too low");

}

</script>

</body>

</html>

The onerror Event

We have just explained how to use the try...catch statement to catch errors in a web page. Now we are going to explain how to use the onerror event for the same purpose.

The onerror event is fired whenever there is a script error in the page.

To use the onerror event, you must create a function to handle the errors. Then you call the function with the onerror event handler. The event handler is called with three arguments: msg (error message), url (the url of the page that caused the error) and line (the line where the error occurred).

Syntax

onerror=handleErr

function handleErr(msg,url,l)

{

//Handle the error here

return true or false

}

The value returned by onerror determines whether the browser displays a standard error message. If you return false, the browser displays the standard error message in the JavaScript console. If you return true, the browser does not display the standard error message.

Example

The following example shows how to catch the error with the onerror event:

<html>

<head>

<script type="text/javascript">

onerror=handleErr;

var txt="";

function handleErr(msg,url,l)

{

txt="There was an error on this page.\n\n";

txt+="Error: " + msg + "\n";

txt+="URL: " + url + "\n";

txt+="Line: " + l + "\n\n";

txt+="Click OK to continue.\n\n";

alert(txt);

return true;

}

function message()

{

adddlert("Welcome guest!");

}

</script>

</head>

<body>

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

</body>

</html>

JavaScript Special Characters

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 Objects Introduction

JavaScript is an Object Oriented Programming (OOP) language.

An OOP language allows you to define your own objects and make your own variable types.

Object Oriented Programming

JavaScript is an Object Oriented Programming (OOP) language. An OOP language allows you to define your own objects and make your own variable types.

However, creating your own objects will be explained later, in the Advanced JavaScript section. We will start by looking at the built-in JavaScript objects, and how they are used. The next pages will explain each built-in JavaScript object in detail.

Note that an object is just a special kind of data. An object has properties and methods.

Properties

Properties are the values associated with an object.

In the following example we are using the length property of the String object to return the number of characters in a string:

<script type="text/javascript">

var txt="Hello World!";

document.write(txt.length);

</script>

Methods

Methods are the actions that can be performed on objects.

In the following example we are using the toUpperCase() method of the String object to display a text in uppercase letters:

<script type="text/javascript">

var str="Hello world!";

document.write(str.toUpperCase());

</script>

String object

The String object is used to manipulate a stored piece of text.

Examples of use:

The following example uses the length property of the String object to find the length of a string:

var txt="Hello world!";

document.write(txt.length);

The code above will result in the following output:

12

The following example uses the toUpperCase() method of the String object to convert a string to uppercase letters:

var txt="Hello world!";

document.write(txt.toUpperCase());

Create a Date Object

The Date object is used to work with dates and times.

The following code create a Date object called myDate:

var myDate=new Date()

Note: The Date object will automatically hold the current date and time as its initial value!

Set Dates

We can easily manipulate the date by using the methods available for the Date object.

In the example below we set a Date object to a specific date (14th January 2010):

var myDate=new Date();

myDate.setFullYear(2010,0,14);

And in the following example we set a Date object to be 5 days into the future:

var myDate=new Date();

myDate.setDate(myDate.getDate()+5);

Note: If adding five days to a date shifts the month or year, the changes are handled automatically by the Date object itself!

Compare Two Dates

The Date object is also used to compare two dates.

The following example compares today's date with the 14th January 2010:

var myDate=new Date();

myDate.setFullYear(2010,0,14);

var today = new Date();

if (myDate>today)

{

alert("Today is before 14th January 2010");

}

else

{

alert("Today is after 14th January 2010");

}

Create an Array

The following code creates an Array object called myCars:

var myCars=new Array();

There are two ways of adding values to an array (you can add as many values as you need to define as many variables you require).

1:

var myCars=new Array();

myCars[0]="Saab";

myCars[1]="Volvo";

myCars[2]="BMW";

You could also pass an integer argument to control the array's size:

var myCars=new Array(3);

myCars[0]="Saab";

myCars[1]="Volvo";

myCars[2]="BMW";

2:

var myCars=new Array("Saab","Volvo","BMW");

Note: If you specify numbers or true/false values inside the array then the type of variables will be numeric or Boolean instead of string.

Access an Array

You can refer to a particular element in an array by referring to the name of the array and the index number. The index number starts at 0.

The following code line:

document.write(myCars[0]);

will result in the following output:

Saab

Modify Values in an Array

To modify a value in an existing array, just add a new value to the array with a specified index number:

myCars[0]="Opel";

Now, the following code line:

document.write(myCars[0]);

Create a Boolean Object

The Boolean object represents two values: "true" or "false".

The following code creates a Boolean object called myBoolean:

var myBoolean=new Boolean();

Note: If the Boolean object has no initial value or if it is 0, -0, null, "", false, undefined, or NaN, the object is set to false. Otherwise it is true (even with the string "false")!

All the following lines of code create Boolean objects with an initial value of false:

var myBoolean=new Boolean();

var myBoolean=new Boolean(0);

var myBoolean=new Boolean(null);

var myBoolean=new Boolean("");

var myBoolean=new Boolean(false);

var myBoolean=new Boolean(NaN);

And all the following lines of code create Boolean objects with an initial value of true:

var myBoolean=new Boolean(true);

var myBoolean=new Boolean("true");

var myBoolean=new Boolean("false");

var myBoolean=new Boolean("Richard");

Math Object

The Math object allows you to perform mathematical tasks.

The Math object includes several mathematical constants and methods.

Syntax for using properties/methods of Math:

var pi_value=Math.PI;

var sqrt_value=Math.sqrt(16);

Note: Math is not a constructor. All properties and methods of Math can be called by using Math as an object without creating it.

Mathematical Constants

JavaScript provides eight mathematical constants that can be accessed from the Math object. These are: E, PI, square root of 2, square root of 1/2, natural log of 2, natural log of 10, base-2 log of E, and base-10 log of E.

You may reference these constants from your JavaScript like this:

Math.E

Math.PI

Math.SQRT2

Math.SQRT1_2

Math.LN2

Math.LN10

Math.LOG2E

Math.LOG10E

Mathematical Methods

In addition to the mathematical constants that can be accessed from the Math object there are also several methods available.

The following example uses the round() method of the Math object to round a number to the nearest integer:

document.write(Math.round(4.7));

The code above will result in the following output:

5

The following example uses the random() method of the Math object to return a random number between 0 and 1:

document.write(Math.random());

The code above can result in the following output:

0.9742574926181294

The following example uses the floor() and random() methods of the Math object to return a random number between 0 and 10:

document.write(Math.floor(Math.random()*11));

More JavaScript Objects

Follow the links to learn more about the objects and their collections, properties, methods and events.

Object

Description

Window

The top level object in the JavaScript hierarchy. The Window object represents a browser window. A Window object is created automatically with every instance of a <body> or <frameset> tag

Navigator

Contains information about the client's browser

Screen

Contains information about the client's display screen

History

Contains the visited URLs in the browser window

Location

Contains information about the current URL

The Navigator Object

The JavaScript Navigator object contains all information about the visitor's browser. We are going to look at two properties of the Navigator object:

appName - holds the name of the browser

appVersion - holds, among other things, the version of the browser

Example

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