JavaScript is used in millions of Web pages to improve the design, validate forms, detect browsers, create cookies, and much more.
JavaScript is the most popular scripting language on the internet, and works in all major browsers, such as Internet Explorer, Mozilla, Firefox, Netscape, Opera.
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)
• A JavaScript consists of lines of executable computer code
• A 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
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Are Java and JavaScript the Same?
NO!
Java and JavaScript are two completely different languages in both concept and design!
Java (developed by Sun Microsystems) is a powerful and much more complex programming language - in the same category as C and C++.
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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("
" + name + "
") 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 will save 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
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How to Put a JavaScript Into an HTML Page
The code above will produce this output on an HTML page:
Hello World!
Example Explained
To insert a JavaScript into an HTML page, we use the tells where the JavaScript starts and ends:
The word document.write is a standard JavaScript command for writing output to a page.
By entering the document.write command between the tags, the browser will recognize it as a JavaScript command and execute the code line. In this case the browser will write Hello World! to the page:
Note: If we had not entered the
The two forward slashes at the end of comment line (//) are a JavaScript comment symbol. This prevents the JavaScript compiler from compiling the line.
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JavaScript Where To ...
JavaScripts in the body section will be executed WHILE the page loads.
JavaScripts in the head section will be executed when CALLED.
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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.
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.
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.
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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
Note: Remember to place the script exactly where you normally would write the script!
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JavaScript Variables________________________________________
Variables
A variable is a "container" for information you want to store. A variable's value can change during the script. You can refer to a variable by name to see its value or to change its value.
Rules for variable names:
• Variable names are case sensitive
• They must begin with a letter or the underscore character
IMPORTANT! JavaScript is case-sensitive! A variable named strname is not the same as a variable named STRNAME!
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Declare a Variable
You can create a variable with the var statement:
var strname = some value
You can also create a variable without the var statement:
strname = some value
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Assign a Value to a Variable
You can assign a value to a variable like this:
var strname = "Hege"
Or like this:
strname = "Hege"
The variable name is on the left side of the expression and the value you want to assign to the variable is on the right. Now the variable "strname" has the value "Hege".
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Lifetime of Variables
When 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.
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JavaScript If...Else Statements
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
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If Statement
You should use the if statement if you want to execute some code only if a specified condition is true.
Syntax
if (condition)
{
code to be executed if condition is true
}
Note that if is written in lowercase letters. Using uppercase letters (IF) will generate a JavaScript error!
Example 1
Example 2
Note: When comparing variables you must always use two equals signs next to each other (==)!
Notice that there is no ..else.. in this syntax. You just tell the code to execute some code only if the specified condition is true.
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If...else Statement
If you want to execute some code if a condition is true and another code if the condition is not true, use the if....else statement.
Syntax
if (condition)
{
code to be executed if condition is true
}
else
{
code to be executed if condition is not true
}
Example
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If...else if...else Statement
You should use the if....else if...else statement if you want to select one of many sets of lines to execute.
Syntax
if (condition1)
{
code to be executed if condition1 is true
}
else if (condition2)
{
code to be executed if condition2 is true
}
else
{
code to be executed if condition1 and
condition2 are not true
}
Example
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JavaScript Switch Statement
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
}
This is how it works: First we have a single expression n (most often a variable), that is evaluated once. The value of the expression is then compared with the values for each case in the structure. If there is a match, the block of code associated with that case is executed. Use break to prevent the code from running into the next case automatically.
Example
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JavaScript Operators
Arithmetic Operators
Operator Description Example Result
+ Addition x=2
y=2
x+y 4
- Subtraction x=5
y=2
x-y 3
* Multiplication x=5
y=4
x*y 20
/ Division 15/5
5/2 3
2.5
% Modulus (division remainder) 5%2
10%8
10%2 1
2
0
++ Increment x=5
x++ x=6
-- Decrement x=5
x-- x=4
Assignment Operators
Operator Example Is The Same As
= x=y x=y
+= x+=y x=x+y
-= x-=y x=x-y
*= x*=y x=x*y
/= x/=y x=x/y
%= x%=y x=x%y
Comparison Operators
Operator Description Example
== is equal to 5==8 returns false
=== is equal to (checks for both value and type) x=5
y="5"
x==y returns true
x===y returns false
!= is not equal 5!=8 returns true
> is greater than 5>8 returns false
< is less than 5<8 returns true
>= is greater than or equal to 5>=8 returns false
<= is less than or equal to 5<=8 returns true
Logical Operators
Operator Description Example
&& and x=6
y=3
(x < 10 && y > 1) returns true
|| or x=6
y=3
(x==5 || y==5) returns false
! not x=6
y=3
!(x==y) returns true
String Operator
A string is most often text, for example "Hello World!". To stick two or more string variables together, use the + operator.
txt1="What a very"
txt2="nice day!"
txt3=txt1+txt2
The variable txt3 now contains "What a verynice day!".
To add a space between two string variables, insert a space into the expression, OR in one of the strings.
txt1="What a very"
txt2="nice day!"
txt3=txt1+" "+txt2
or
txt1="What a very "
txt2="nice day!"
txt3=txt1+txt2
The variable txt3 now contains "What a very nice day!".
Conditional Operator
JavaScript also contains a conditional operator that assigns a value to a variable based on some condition.
Syntax
variablename=(condition)?value1:value2
Example
greeting=(visitor=="PRES")?"Dear President ":"Dear "
If the variable visitor is equal to PRES, then put the string "Dear President " in the variable named greeting. If the variable visitor is not equal to PRES, then put the string "Dear " into the variable named greeting.
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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")
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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")
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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
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JavaScript Functions
To keep the browser from executing a script as soon as the page is loaded, you can write your script as a function.
A function contains some code that will be executed only 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 are defined at the beginning of a page, in the section.
Example
If the line: alert("Hello world!!"), in the example above had not been written within a function, it would have been executed as soon as the line was loaded. Now, the script is not executed before the user hits the button. We have added an onClick event to the button that will execute the function displaymessage() when the button is clicked.
You will learn more about JavaScript events in the JS Events chapter.
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How to Define a Function
The syntax for creating a function is:
function functionname(var1,var2,...,varX)
{
some code
}
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:
function functionname()
{
some code
}
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.
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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 total(a,b)
{
x=a*b
return x
}
When you call the function above, you must pass along two parameters:
product=total(2,3)
The returned value from the total() function is 6, and it will be stored in the variable called product.
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JavaScript For Loop
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 is 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
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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.
Result
The number is 0
The number is 1
The number is 2
The number is 3
The number is 4
The number is 5
The number is 6
The number is 7
The number is 8
The number is 9
The number is 10
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The while loop
The while loop will be explained in the next chapter.
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JavaScript While Loop
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.
Result
The number is 0
The number is 1
The number is 2
The number is 3
The number is 4
The number is 5
The number is 6
The number is 7
The number is 8
The number is 9
The number is 10
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 once, even if the condition is false, because the code are executed before the condition is tested.
do
{
code to be executed
}
while (var<=endvalue)
Example
Result
The number is 0
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JavaScript Browser Detection
The JavaScript 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 - specially on older browsers.
So, sometimes it can be very useful to detect the visitor's browser type and version, and then serve up the appropriate information.
The best way to do this is to make your web pages smart enough to look one way to some browsers and another way to other browsers.
JavaScript includes an object called the Navigator object, that can be used for this purpose.
The Navigator object contains information about the visitor's browser name, browser version, and more.
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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
The variable browser in the example above holds the name of the browser, i.e. "Netscape" or "Microsoft Internet Explorer".
The appVersion property in the example above returns a string that contains much more information than just the version number, but for now we are only interested in the version number. To pull the version number out of the string we are using a function called parseFloat(), which pulls the first thing that looks like a decimal number out of a string and returns it.
IMPORTANT! The version number is WRONG in IE 5.0 or later! Microsoft start the appVersion string with the numbers 4.0. in IE 5.0 and IE 6.0!!! Why did they do that??? However, JavaScript is the same in IE6, IE5 and IE4, so for most scripts it is ok.
Example
The script below displays a different alert, depending on the visitor's browser:
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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
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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,expiredays)
{
var exdate=new Date()
exdate.setDate(expiredays)
document.cookie=c_name+ "=" +escape(value)+
((expiredays==null) ? "" : ";expires="+exdate)
}
The parameters of the function above holds 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 checks if the cookie has been set:
function getCookie(c_name)
{
if (document.cookie.length>0)
{
c_start=document.cookie.indexOf(c_name + "=")
if (c_start!=-1)
{
c_start=c_start + c_name.length+1
c_end=document.cookie.indexOf(";",c_start)
if (c_end==-1) c_end=document.cookie.length
return unescape(document.cookie.substring(c_start,c_end))
}
}
return null
}
The function above first checks if a cookie is stored at all in the document.cookie object. If the document.cookie object holds some cookies, then check to see if our specific cookie is stored. If our cookie is found, then return the value, if not - return null.
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:
function checkCookie()
{
username=getCookie('username')
if (username!=null)
{alert('Welcome again '+username+'!')}
else
{
username=prompt('Please enter your name:',"")
if (username!=null||username!="")
{
setCookie('username',username,365)
}
}
}
All together now:
The example above runs the checkCookie() function when the page loads.
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JavaScript Form Validation JavaScript can be used to validate input data in HTML forms before sending off the content to a server.
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JavaScript Form Validation
JavaScript can be used to validate input 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?
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Required Fields
The function below checks if a required field has been left empty. If the required field is blank, an alert box alerts a message and the function returns false. If a value is entered, the function returns true (means that data is OK):
function validate_required(field,alerttxt)
{
with (field)
{
if (value==null||value=="")
{alert(alerttxt);return false}
else {return true}
}
}
The entire script, with the HTML form could look something like this:
onsubmit="return validate_form(this)"
method="post">
Email:
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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 at least an @ sign and a dot (.). Also, the @ must not be the first character of the email address, and the last dot must at least be one character after the @ sign:
function validate_email(field,alerttxt)
{
with (field)
{
apos=value.indexOf("@")
dotpos=value.lastIndexOf(".")
if (apos<1||dotpos-apos<2)
{alert(alerttxt);return false}
else {return true}
}
}
The entire script, with the HTML form could look something like this:
onsubmit="return validate_form(this)"
method="post">
Email:
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JavaScript Animation
JavaScript Animation
It is possible to use JavaScript to create animated images.
The trick is to let a JavaScript change between different images on different events.
In the following example we will add an image that should act as a link button on a web page. We will then add an onMouseOver event and an onMouseOut event that will run two JavaScript functions that will change between the images.
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The HTML Code
The HTML code looks like this:
onmouseOver="mouseOver()"
onmouseOut="mouseOut()">
Note that we have given the image a name to make it possible for JavaScript to address it later.
The onMouseOver event tells the browser that once a mouse is rolled over the image, the browser should execute a function that will replace the image with another image.
The onMouseOut event tells the browser that once a mouse is rolled away from the image, another JavaScript function should be executed. This function will insert the original image again.
IMPORTANT! The mouse events are added to the tag, and not to the
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The JavaScript Code
The changing between the images is done with the following JavaScript:
The function mouseOver() causes the image to shift to "b_blue.gif".
The function mouseOut() causes the image to shift to "b_pink.gif".
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The Entire Code
onmouseOver="mouseOver()"
onmouseOut="mouseOut()">
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Reference:
http://www.w3schools.com/
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