Java Script Faq's &

JavaScript Tutorial

2009-01-02T02:22:00.000-08:00

JavaScript is the scripting language of the Web!

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.

JavaScript is easy to learn! You will enjoy it!

Start learning JavaScript now!

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 You Should Already Know

Before you continue you should have a basic understanding of the following:

HTML / XHTML

If you want to study these subjects first, find the tutorials on our Home page.

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

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++.

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

The HTML

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.

JavaScripts in the body section will be executed WHILE the page loads.

JavaScripts in the head section will be executed when CALLED.

Examples

Head section
Scripts that contain functions go in the head section of the document. Then we can be sure that the script is loaded before the function is called.

Body section
Execute a script that is placed in the body section.

External script
How to access an external script.

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.

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!

A variable is a "container" for information you want to store.

Examples

Variable
Variables are used to store data. This example will show you how.

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!

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

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".

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.

Conditional statements in JavaScript are used to perform different actions based on different conditions.

Examples

If statement
How to write an if statement.

If...else statement
How to write an if...else statement.

If..else if...else statement
How to write an if..else if...else statement.

Random link
This example demonstrates a link, when you click on the link it will take you to W3Schools.com OR to RefsnesData.no. There is a 50% chance for each of them.

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

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.

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

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

Conditional statements in JavaScript are used to perform different actions based on different conditions.

Examples

Switch statement
How to write a 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

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>
>=	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 <> 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.

In JavaScript we can create three kind of popup boxes: Alert box, Confirm box, and Prompt box.

Examples

Alert box

Alert box with line breaks

Confirm box

Prompt box

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")

A function is a reusable code-block that will be executed by an event, or when the function is called.

Examples

Function
How to call a function.

Function with arguments
How to pass a variable to a function, and use the variable in the function.

Function with arguments 2
How to pass variables to a function, and use these variables in the function.

Function that returns a value
How to let the function return a value.

A function with arguments, that returns a value
How to let the function find the product of 2 arguments and return the result.

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

onclick="displaymessage()" >

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.

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.

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

When you call the function above, you must pass along two parameters:

product=prod(2,3)

The returned value from the prod() function is 6, and it will be stored in the variable called product.

Loops in JavaScript are used to execute the same block of code a specified number of times or while a specified condition is true.

Examples

For loop
How to write a for loop. Use a For loop to run the same block of code a specified number of times.

Looping through HTML headers
How to use the for loop to loop through the different HTML headers.

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.

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 while loop

The while loop will be explained in the next chapter.

Loops in JavaScript are used to execute the same block of code a specified number of times or while a specified condition is true.

Examples

While loop
How to write a while loop. Use a while loop to run the same block of code while a specified condition is true.

Do while loop
How to write a do...while loop. Use a do...while loop to run the same block of code while a specified condition is true. This loop will always be executed at least once, even if the condition is false, because the statements are executed before the condition is tested.

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 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

Result

The number is 0

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

Examples

Break statement
Use the break statement to break the loop.

Continue statement
Use the continue statement to break the current loop and continue with the next value.

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

Result

The number is 0

The number is 1

The number is 2

Continue

The continue command will break the current loop and continue with the next value.

Example

Result

The number is 0

The number is 1

The number is 2

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

Examples

For...In statement
How to use a for...in statement to loop through the elements of an array.

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:

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 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

The following table lists the events recognized by JavaScript:

Note: Events are normally used in combination with functions, and the function will not be executed before the event occurs!

For a complete reference of the events recognized by JavaScript, go to our complete Event reference.

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:

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:

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:

onmouseover="alert('An onMouseOver event');return false">

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

Examples

The try...catch statement
How to write a try...catch statement.

The try...catch statement with a confirm box
Another example of how to write a try...catch statement.

JavaScript - Catching Errors

When browsing Web pages on the internet, I guess we have all seen a JavaScript alert box, telling you there is a runtime error, and asking: "Do you wish to debug?" on some pages. Error message like that may be useful for developers, but not for the 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:

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:

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:

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.

The throw statement allows you to create an exception.

Examples

The throw statement
How to use the throw statement.

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:

Using the onerror event is the old standard solution to catch errors in a web page.

Examples

The onerror event
How to use the onerror event to catch errors in a web page.

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:

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)

JavasScript will now output the proper text string: We are the so-called "Vikings" from the north.

Here is another example:

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

The example above will produce the following output:

You & me 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

Some other important things to know when scripting with JavaScript.

JavaScript is Case Sensitive

A function named "myfunction" is not the same as "myFunction" and a variable named "myVar" is not the same as "myvar".

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

White Space

JavaScript ignores extra spaces. You can add white space to your script to make it more readable. The following lines are equivalent:

name="Hege"

name = "Hege"

Break up a Code Line

You can break up a code line within a text string with a backslash. The example below will be displayed properly:

document.write("Hello \

World!")

However, you cannot break up a code line like this:

document.write \

("Hello World!")

Comments

You can add comments to your script by using two slashes //:

//this is a comment

document.write("Hello World!")

or by using /* and */ (this creates a multi-line comment block):

/* This is a comment

block. It contains

several lines */

document.write("Hello World!")

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:

The output of the code above will be:

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:

The output of the code above will be:

HELLO WORLD!

Examples

Return the length of a string
How to use the length property to find the length of a string.

Style strings
How to style strings.

The indexOf() method
How to use the indexOf() method to return the position of the first occurrence of a specified string value in a string.

The match() method
How to use the match() method to search for a specified string value within a string and return the string value if found

Replace characters in a string - replace()
How to use the replace() method to replace some characters with some other characters in a string.

Tip: You will find a lot more examples in the String object reference - look at the bottom of this page!

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:

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())

The code above will result in the following output:

HELLO WORLD!

Complete String Object Reference

For a complete reference of all the properties and methods that can be used with the String object, go to our complete String object reference.

The reference contains a brief description and examples of use for each property and method!

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

Examples

Return today's date and time
How to use the Date() method to get today's date.

getTime()
Use getTime() to calculate the years since 1970.

setFullYear()
How to use setFullYear() to set a specific date.

toUTCString()
How to use toUTCString() to convert today's date (according to UTC) to a string.

getDay()
Use getDay() and an array to write a weekday, and not just a number.

Display a clock
How to display a clock on your web page.

Tip: You will find a lot more examples in the Date object reference - look at the bottom of this page!

Defining Dates

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

We define a Date object with the new keyword. The following code line defines 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!

Manipulate 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!

Comparing 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")

Complete Date Object Reference

For a complete reference of all the properties and methods that can be used with the Date object, go to our complete Date object reference.

The reference contains a brief description and examples of use for each property and method!

The Array object is used to store a set of values in a single variable name.

Examples

Create an array
Create an array, assign values to it, and write the values to the output.

For...In Statement
How to use a for...in statement to loop through the elements of an array.

Join two arrays - concat()
How to use the concat() method to join two arrays.

Put array elements into a string - join()
How to use the join() method to put all the elements of an array into a string.

Literal array - sort()
How to use the sort() method to sort a literal array.

Numeric array - sort()
How to use the sort() method to sort a numeric array.

Tip: You will find a lot more examples in the Array object reference - look at the bottom of this page!

Defining Arrays

The Array object is used to store a set of values in a single variable name.

We define an Array object with the new keyword. The following code line defines an Array object called myArray:

var myArray=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).

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"

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.

Accessing Arrays

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 Existing Arrays

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])

will result in the following output:

Opel

Complete Array Object Reference

For a complete reference of all the properties and methods that can be used with the Array object, go to our complete Array object reference.

The reference contains a brief description and examples of use for each property and method!

The Boolean object is used to convert a non-Boolean value to a Boolean value (true or false).

Examples

Check Boolean value
Check if a Boolean object is true or false.

Tip: You will find more examples in the Boolean object reference - look at the bottom of this page!

Boolean Object

The Boolean object is an object wrapper for a Boolean value.

The Boolean object is used to convert a non-Boolean value to a Boolean value (true or false).

We define a Boolean object with the new keyword. The following code line defines 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")

Complete Boolean Object Reference

For a complete reference of all the properties and methods that can be used with the Boolean object, go to our complete Boolean object reference.

The reference contains a brief description and examples of use for each property and method!

The Math object allows you to perform common mathematical tasks.

Examples

round()
How to use round().

random()
How to use random() to return a random number between 0 and 1.

max()
How to use max() to return the number with the highest value of two specified numbers.

min()
How to use min() to return the number with the lowest value of two specified numbers.

Tip: You will find a lot more examples in the Math object reference - look at the bottom of this page!

Math Object

The Math object allows you to perform common mathematical tasks.

The Math object includes several mathematical values and functions. You do not need to define the Math object before using it.

Mathematical Values

JavaScript provides eight mathematical values (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 values 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 values that can be accessed from the Math object there are also several functions (methods) available.

Examples of functions (methods):

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:

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:

0.9425462674836698

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))

The code above can result in the following output:

Complete Math Object Reference

For a complete reference of all the properties and methods that can be used with the Math object, go to our complete Math object reference.

The reference contains a brief description and examples of use for each property and method!

In addition to the built-in JavaScript objects, you can also access and manipulate all of the HTML DOM objects with JavaScript.

The HTML DOM

The HTML DOM is a W3C standard and it is an abbreviation for the Document Object Model for HTML.

The HTML DOM defines a standard set of objects for HTML, and a standard way to access and manipulate HTML documents.

All HTML elements, along with their containing text and attributes, can be accessed through the DOM. The contents can be modified or deleted, and new elements can be created.

The HTML DOM is platform and language independent. It can be used by any programming language like Java, JavaScript, and VBScript.

Follow the links below to learn more about how to access and manipulate each DOM object with JavaScript:

Object	Description
Anchor	Represents an HTML a element (a hyperlink)
Applet	Represents an HTML applet element. The applet element is used to place executable content on a page
Area	Represents an area of an image-map. An image-map is an image with clickable regions
Base	Represents an HTML base element
Basefont	Represents an HTML basefont element
Body	Represents the body of the document (the HTML body)
Button	Represents a push button on an HTML form. For each instance of an HTML tag on an HTML form, a Button object is created
Checkbox	Represents a checkbox on an HTML form. For each instance of an HTML tag on an HTML form, a Checkbox object is created
Document	Used to access all elements in a page
Event	Represents the state of an event, such as the element in which the event occurred, the state of the keyboard keys, the location of the mouse, and the state of the mouse buttons
FileUpload	For each instance of an HTML tag on a form, a FileUpload object is created
Form	Forms are used to prompt users for input. Represents an HTML form element
Frame	Represents an HTML frame
Frameset	Represents an HTML frameset
Hidden	Represents a hidden field on an HTML form. For each instance of an HTML tag on a form, a Hidden object is created
History	A predefined object which can be accessed through the history property of the Window object. This object consists of an array of URLs. These URLs are all the URLs the user has visited within a browser window
Iframe	Represents an HTML inline-frame
Image	Represents an HTML img element
Link	Represents an HTML link element. The link element can only be used within the tag
Location	Contains information about the current URL
Meta	Represents an HTML meta element
Navigator	Contains information about the client browser
Option	Represents an option in a selection list on an HTML form. For each instance of an HTML
Password	Represents a password field on an HTML form. For each instance of an HTML tag on a form, a Password object is created
Radio	Represents radio buttons on an HTML form. For each instance of an HTML tag on a form, a Radio object is created
Reset	Represents a reset button on an HTML form. For each instance of an HTML tag on a form, a Reset object is created
Screen	Automatically created by the JavaScript runtime engine and it contains information about the client's display screen
Select	Represents a selection list on an HTML form. For each instance of an HTML
Style	Represents an individual style statement. This object can be accessed from the document or from the elements to which that style is applied
Submit	Represents a submit button on an HTML form. For each instance of an HTML tag on a form, a Submit object is created
Table	Represents an HTML table element
TableData	Represents an HTML td element
TableHeader	Represents an HTML th element
TableRow	Represents an HTML tr element
Text	Represents a text field on an HTML form. For each instance of an HTML tag on a form, a Text object is created
Textarea	Represents an HTML textarea element
Window	Corresponds to the browser window. A Window object is created automatically with every instance of a or tag

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

Examples

Detect the visitor's browser and browser version

More details about the visitor's browser

All details about the visitor's browser

Alert user, depending on 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.

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 starts the appVersion string with the number 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:

A cookie is often used to identify a user.

Examples

Create a welcome cookie

What is a Cookie?

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

Examples of cookies:

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

Create and Store a Cookie

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

First, we create a function that stores the name of the visitor in a cookie variable:

function setCookie(c_name,value,expiredays)

{

var exdate=new Date()

exdate.setDate(exdate.getDate()+expiredays)

document.cookie=c_name+ "=" +escape(value)+

((expiredays==null) ? "" : ";expires="+exdate)

The parameters of the function above hold the name of the cookie, the value of the cookie, and the number of days until the cookie expires.

In the function above we first convert the number of days to a valid date, then we add the number of days until the cookie should expire. After that we store the cookie name, cookie value and the expiration date in the document.cookie object.

Then, we create another function that 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.

JavaScript can be used to validate input data in HTML forms before sending off the content to a server.

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?

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:

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:

With JavaScript we can create animated images.

Examples

Button 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.

The HTML Code

The HTML code looks like this:

onmouseOver="mouseOver()"
onmouseOut="mouseOut()">src="b_pink.gif" name="b1" />

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 tag. Unfortunately, browsers do not support mouse events on images!

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".

The Entire Code

onmouseOver="mouseOver()"
onmouseOut="mouseOut()">src="b_pink.gif" name="b1" />

An image-map is an image with clickable regions.

Examples

Simple HTML Image map

Image map with added JavaScript

HTML Image Maps

From our HTML tutorial we have learned that an image-map is an image with clickable regions. Normally, each region has an associated hyperlink. Clicking on one of the regions takes you to the associated link.

Example

The example below demonstrates how to create an HTML image map, with clickable regions. Each of the regions is a hyperlink:

width ="145" height ="126"
alt="Planets"
usemap ="#planetmap" />
name="planetmap">
  href ="sun.htm" target ="_blank"
  alt="Sun" />
  href ="mercur.htm" target ="_blank"
  alt="Mercury" />
  href ="venus.htm" target ="_blank"
  alt="Venus" />

Result

Adding some JavaScript

We can add events (that can call a JavaScript) to the tags inside the image map. The tag supports the onClick, onDblClick, onMouseDown, onMouseUp, onMouseOver, onMouseMove, onMouseOut, onKeyPress, onKeyDown, onKeyUp, onFocus, and onBlur events.

Here's the above example, with some JavaScript added:

alt="Planets" usemap="#planetmap" />
 
onMouseOver="writeText('The Sun and the gas giant
planets like Jupiter are by far the largest objects
in our Solar System.')"
href ="sun.htm" target ="_blank" alt="Sun" />
 
onMouseOver="writeText('The planet Mercury is very
difficult to study from the Earth because it is
always so close to the Sun.')"
href ="mercur.htm" target ="_blank" alt="Mercury" />
 
onMouseOver="writeText('Until the 1960s, Venus was
often considered a twin sister to the Earth because
Venus is the nearest planet to us, and because the
two planets seem to share many characteristics.')"
href ="venus.htm" target ="_blank" alt="Venus" />

With JavaScript, it is possible to execute some code NOT immediately after a function is called, but after a specified time interval. This is called timing events.

Examples

Simple timing

Another simple timing

Timing event in an infinite loop

Timing event in an infinite loop - with a Stop button

A clock created with a timing event

JavaScript Timing Events

With JavaScript, it is possible to execute some code NOT immediately after a function is called, but after a specified time interval. This is called timing events.

It's very easy to time events in JavaScript. The two key methods that are used are:

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

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

setTimeout()

Syntax

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

The setTimeout() method returns a value - In the statement above, the value is stored in a variable called t. If you want to cancel this setTimeout(), you can refer to it using the variable name.

The first parameter of setTimeout() is a string that contains a JavaScript statement. This statement could be a statement like "alert('5 seconds!')" or a call to a function, like "alertMsg()".

The second parameter indicates how many milliseconds from now you want to execute the first parameter.

Note: There are 1000 milliseconds in one second.

Example

When the button is clicked in the example below, an alert box will be displayed after 5 seconds.

onClick="timedMsg()">

Example - Infinite Loop

To get a timer to work in an infinite loop, we must write a function that calls itself. In the example below, when the button is clicked, the input field will start to count (for ever), starting at 0:

onClick="timedCount()">

clearTimeout()

Syntax

clearTimeout(setTimeout_variable)

Example

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

onClick="timedCount()">
onClick="stopCount()">

Objects are useful to organize information.

Examples

Create a direct instance of an object

Create a template for an object

JavaScript Objects

Earlier in this tutorial we have seen that JavaScript has several built-in objects, like String, Date, Array, and more. In addition to these built-in objects, you can also create your own.

An object is just a special kind of data, with a collection of properties and methods.

Let's illustrate with an example: A person is an object. Properties are the values associated with the object. The persons' properties include name, height, weight, age, skin tone, eye color, etc. All persons have these properties, but the values of those properties will differ from person to person. Objects also have methods. Methods are the actions that can be performed on objects. The persons' methods could be eat(), sleep(), work(), play(), etc.

Properties

The syntax for accessing a property of an object is:

objName.propName

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

personObj.firstname="John"

personObj.lastname="Doe"

personObj.age=30

personObj.eyecolor="blue"

document.write(personObj.firstname)

The code above will generate the following output:

John

Methods

An object can also contain methods.

You can call a method with the following syntax:

objName.methodName()

Note: Parameters required for the method can be passed between the parentheses.

To call a method called sleep() for the personObj:

personObj.sleep()

Creating Your Own Objects

There are different ways to create a new object:

1. Create a direct instance of an object

The following code creates an instance of an object and adds four properties to it:

personObj=new Object()

personObj.firstname="John"

personObj.lastname="Doe"

personObj.age=50

personObj.eyecolor="blue"

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

personObj.eat=eat

2. Create a template of an object

The template defines the structure of an object:

function person(firstname,lastname,age,eyecolor)

this.firstname=firstname

this.lastname=lastname

this.age=age

this.eyecolor=eyecolor

Notice that the template is just a function. Inside the function you need to assign things to this.propertyName. The reason for all the "this" stuff in is that you're going to have more than one person at a time (which person you're dealing with must be clear). That's what "this" is: the instance of the object at hand.

Once you have the template, you can create new instances of the object, like this:

myFather=new person("John","Doe",50,"blue")

myMother=new person("Sally","Rally",48,"green")

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

function person(firstname,lastname,age,eyecolor)

this.firstname=firstname

this.lastname=lastname

this.age=age

this.eyecolor=eyecolor

this.newlastname=newlastname

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

function newlastname(new_lastname)

this.lastname=new_lastname

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

JavaScript Summary

This tutorial has taught you how to add JavaScript to your HTML pages, to make your web site more dynamic and interactive.

You have learned how to create responses to events, validate forms and how to make different scripts run in response to different scenarios.

You have also learned how to create and use objects, and how to use JavaScript's built-in objects.

For more information on JavaScript, please look at our JavaScript examples and our JavaScript reference.

Javascript Faqs

2008-10-20T00:35:00.001-07:00

Q. What is JavaScript?
A. JavaScript is a scripting language designed for adding interactivity to HTML pages. JavaScript is an interpreted language. This means that scripts execute without preliminary compilation, i.e. without conversion of the script text into a system-dependent machine code. The user's browser interprets the script, that is, analyzes and immediately executes it.

Thus, most Internet users today have browsers that support JavaScript. That's why JavaScript is one of the most popular tools for adding interactive features to Web pages.

Q: How do I insert comments in JavaScript code?
A: JavaScript supports three different types of comments:

1. Multiple-line C-style comments. Everything between /* and */ is a comment,
for example:
/* This is a comment */
/* C-style comments can span
as many lines as you like,
as shown in this example */

2. One-line comments of C++ style. These comments begin with // and continue up to the next line break:
// This is a one-line comment

3. One-line comments with the HTML comment-opening sequence (). Consider this example:
This is also a one-line JS comment
because JS ignores the closing characters
of HTML-style comments

Q: How do I hide JS code from old browsers that do not support JavaScript?
A: To prevent old browsers from displaying your JS code, do the following:

1) Immediately after the opening

Thus, your HTML file will contain the following fragment:

Q: If the user's browser cannot execute JavaScript code, can I display a warning for the user?
A: Yes, you can display a special warning for users of JavaScript-incapable browsers. Put your warning text between the tags .
Here's an example:

JavaScript-enabled browsers will ignore everything between . Browsers that cannot execute JavaScript will display your message on the page.

Q: Can I include JavaScript code from external JS files, rather than embedding all my scripts within HTML pages?
Answer: Yes. To do so, create a new file with the extension .js, for example, myscript.js. Put your JavaScript code in this file; do not include opening and closing SCRIPT tags in the .js file!

To embed myscript.js into your Web page, use these tags in your HTML file:

Q: Can I make a button on my page work as the browser's Back button?
A: To create your own Back button, use this code

Q: Can I make a button on my page work as the browser's Forward button?
A: To create your own Forward button, use this code

If your browser's Forward button is currently inactive, then the Forward button on your page won't work either. This is the case when the current page is the last page in your browsing history.

Q: Can I pass parameters from one my page to another?
Answer: Yes, you can pass a parameter to another page in several different ways:

by storing the parameter in a cookie
by storing the parameter in a variable of another window or frame
by storing the parameter in the rewritable property top.name (the browser window name)
by appending the parameter to the destination page's URL as a query string

Q: How do I convert strings to numbers in JavaScript?
Answer: To convert a string to a number, use the JavaScript function parseFloat (for conversion to a floating-point number) or parseInt (for conversion to an integer).

parseFloat syntax: parseFloat('string')

Q: Is there a way to test whether a particular variable holds a number or a string?
Answer: Yes. To test whether the variable holds a number or a string, use the typeof operator. If your variable holds a number, typeof(variable) will return "number". If it holds a string, typeof(variable) will return "string". The following are examples of typeof usage:

typeof(123) // result: "number"
typeof("123") // result: "string"

if (typeof k == "string") { alert('k is a string.') }
if (typeof k == "number") { alert('k is a number.') }

Q: How do I generate random numbers in JavaScript?
Answer: To generate random floating-point numbers in the range from 0 to 1, use the Math.random() method:

num = Math.random() // num is random, from 0 to 1

If you need random floating-point numbers in the range from A to B, use this code:
num = A + (B-A)*Math.random() // num is random, from A to B

Q: How do I extract a substring from a string?
Answer: To extract a substring from a string, use the substring method:

string.substring(start,end)

Here
string - is the string from which you want to extract a substring.
start - is the number specifying the position of the character at which the substring begins. (The character at start itself will be included in the substring.)
end - is the number specifying the position of the character at which the substring ends. (The character at end will not be included in the substring.)

Note that the first character in the string corresponds to position 0, and the last character to position string.length-1.

Examples:

'Hello'.substring(0,2) // 'He'
'Hello'.substring(0,4) // 'Hell'

Q: Can I play a sound file without using JavaScript?
A: Yes, you can play a sound by specifying the sound file's URL as a hyperlink destination, for example,

Q: How do I set a background sound on a Web page?
A: In all audio-enabled browsers, you can use the EMBED tag to play a background sound. For example, if you want to play the file bkground.mid right after the browser loads the page, you can use the following EMBED tag:

To stop the background sound,call the cross-browser method document.bkgroundID.stop() .

If your target browser is Microsoft Internet Explorer (for example, in an intranet), then you can use the Explorer-specific BGSOUND tag:

Here, again, bkground.mid stands for the name of the sound file that you actually want to play.

Q: How do I generate a user prompt from JavaScript?
A: To generate a user prompt, use the prompt() method:

prompt('Prompt text','Suggested input')

use the following code:

Q: What value does prompt() return if the user clicked the Cancel button?
A: The return value of canceled prompt() depends on the browser. In some browsers the return value is null, in others it's '' (empty string). Therefore, you might want to use the following code when calling the prompt() method:

userInput = prompt('Prompt text','Suggested input');
if (userInput != '' && userInput != null) {
// do something with the input
}

Q: How do I disable a radio button in a form (making it not selectable)?
A: To make a radio button not selectable, in the button's INPUT tag you can use an onClick event handler like this:

Q: How do I change an image when the user's mouse points at it?
A: Here is a simple example:
Point at this folder, and it will open. Move the mouse away, and the folder will close.

In this example, the image is 2.gif; the image is 1.gif. Both files are stored in the ../hi-icons directory. In order to create the "mouseover" effect, the tag is embedded in a hyperlink that has onMouseOver and onMouseOut event handlers:

In the section of the page, we have JavaScript code that preloads the image files and defines the event handler functions:

Q: How do I check whether the user clicked the left or right mouse button?
A: The click event occurs for the left mouse button only. Therefore, onClick event handlers do not need to preform the left-versus-right button test.

On the other hand, the mousedown and mouseup events may occur for any mouse button. To determine whether the user clicked the left or right button, you can use the following event properties:

event.which in Netscape Navigator
event.button in Internet Explorer

If the value of these properties is 1, the event occurred for the left button. In the following example, the onMouseDown event handler displays the messages Left button or Right button, depending on the mouse button you actually have clicked. The messages will appear on your browser's status bar. Click or right-click anywhere on this page to see it work:

Q: Can I disable the Windows context menu that normally shows up when the user clicks the right mouse button?
A: In most modern browsers, you can disable the context menu by using the oncontextmenu event handler in the body tag of your page:

Q: Can I disable the right-click context menu for a particular image, while still enabling the menu for all other parts of my page?
Answer: Yes. In most modern browsers you can disable the right-click menu for a particular image. To do so, you can use the event handler oncontextmenu="return false;" within the IMG tag that defines your image:

Q: How do I determine the week day for a given date?
A: To determine the week day for a given date, you set this date in a Date() object, and then get the week day using the Date.getDay() method:

d=new Date();
d.setDate(1);

d.setYear(yyyy);
d.setMonth(mm);
d.setDate(dd);

ww=d.getDay();
if (ww==0) wDay="Sunday";
if (ww==1) wDay="Monday";
if (ww==2) wDay="Tuesday";
if (ww==3) wDay="Wednesday";
if (ww==4) wDay="Thursday";
if (ww==5) wDay="Friday";
if (ww==6) wDay="Saturday";

Q: How can I show a different image each day?
A:
we can display a different image for each day of the week:

This creates a date object and then gets the number of the day of the week, and then uses that to display the relevant image.

If you have more than 7 images then you'll need to alter this, either using the day of the month or the day of the year, or maybe even the day of the millennium!

Q: How do I display a random banner image?
A:There is an existing JavaScript method that returns a random number:

//Returns a random number between 0 and 1
function getRandom() {
return Math.random()
}

In Netscape Navigator 2 this only works on Unix platforms, so you might want to try the following instead:

To create a random number between 1 and 10 use rand(10). To display an random image named banner1.gif to banner10.gif use:

Q: How do I disable the "Back" button of a browser?
Answer:

Easy answer - you can't.

Longer answer - you cannot totally remove the ability for the user to go back to the previous location, although you can make it harder.

1) You can use the location replace method when changing the location. This replaces the current history location with the new location. However older browsers do not support this method, which is why it is required to test for the images object:

Note: in Opera replace() will work, but only with absolute URL's.

2) You can load the new location into another window, and then close the old window:

In the first page:

Open Window

In the next window:

change location

3) You can open a window without a toolbar:

However, this does not totally stop the user from being able to go back to the previous page.

4) You can add code to the previous page to force the browser to go forwards again:

Q: What's the difference between Java and JavaScript and why should I prevent my browser from executing JavaScript?
A:
JavaScript is an OO type interpreted language that runs on the clients browser, enabling interaction between the user and the document, without the need to always go back to the server for processing. for example to validate form input. There is no reason why you would want to stop JavaScript from executing on your browser, unless yet another security hole is found in the browser that allows data to be captured by a third party. All the *known* security holes have been plugged.

Java is a byte compiled language that is a fully fledged OO language. It again runs on the clients browser. It can do things that JavaScript cannot do, like reading a file on the same server that the document was served from (I believe this is correct). It can do graphcs. I'm not too sure if there are valid reasons why you would not want to allow Java to execute on your browser, other than that Java applets can be quite big and can take a while to download. The initial loading of Java in the browser can take quite a few seconds to start up.

Q: Is there a way to close a window without that confirm message?
Answer:
The only way to avoid the message appearing is when you close a window that only has one location in its history. Most visitors to a site will have many locations in their current history.

If you are creating a window that you then want to close later on without the message occurring, then make sure that you use the JavaScript replace() method when changing the location within that window, that way you will not create new entries within that windows history object.

To use the replace method without causing errors on browsers that don't support it you'll need to do the following:

And then for all your links in that window:

Next Page

Make sure you use the absolute URL, because in some versions of Opera, an absolute reference will cause problems when used with the replace() method.

Note that for browsers that don't support the replace method (the same as those that don't support the image object) the window will build up entries in the history object.

Q: How can I disable the "View Source" menu item?
A:
Sorry, not possible.

Q: How can I calculate the number of days elapsed between two dates?
A:

Q: How do you subtract X hours from a date?
A:

The following subtracts five hours from the current date and time:

Q: How can I calculate the difference between two dates?
Answer:

Q: How can I calculate the number of days in the current month?
A:
The following little gem was provided by Paul Bennett. At first look it looks too simple, but on closer investigation it is a pretty clever piece of code:

function days_in_month (year, month) {
return 32 - new Date(year, month, 32).getDate();
}

Q: How can I convert a text string: "Wed Aug 26 14:40:45 MET DST 1998." into a Date object?
Answer:
The date foramt has to be altered to:

Wed, 26 Aug 1998 14:40:45So:

Q: How can I convert a mm/dd/yy date to dd/mm/yy and vice versa?
Answer:

Q: How do I add a number of days to a date?
Answer:

Q: How can I cancel the users F5 request to refresh the page in Internet Explorer?
Answer:

Do it by redirecting the keyboard action on another keycode that you can control (backspace in this example):

function cancelRefresh() {
// keycode for F5 function
if (window.event && window.event.keyCode == 116) {
window.event.keyCode = 8;
}
// keycode for backspace
if (window.event && window.event.keyCode == 8) {
// try to cancel the backspace
window.event.cancelBubble = true;
window.event.returnValue = false;
return false; }}

This function is called on the onKeyDown event and it seems to work!

Q: When creating a form, how can I check to see if data entered in a text field contains all numerics?
A:

Q: How do I get a word count of the text in a textarea?
Answer:

Q: When a page reloads (due to the user pressing back) how do I clear a form of all the user entered data?
Answer:

Simply invoke the form's reset method, immediately after defining the form, for example:

Q: How can I extract just the extension file name from a forms file upload field?
Answer:

Q: How can I check where a visitor has come from?
A:

Q: How can you make a .wav play when you click on an image?
A:

You should use the tag.

The following will play the midi or wav file as soon as its loaded on a Java enabled browser:

The following allows you control when it plays:

You can replace sound.wav with sound.mid.

Q: When I open a new window, how can I change the contents of the original window from the new window?
A:

Within the original window:

Within the new window:

load document into opener

load document

Q: How do I open a window at a given position on the screen?
A:
In Netscape Navigator 4 the screenX and screenY attributes were introduced. In Internet Explorer 4 the top and left attributes were introduced. By combining the two both Netscape Navigator 4 and Internet Explorer 4 will allow you to position the window:

Q: Is it possible to close the parent window (main web browser window) from a child window?
A:

Put the following in the child:

Although you'll need to make sure that the opener has been defined for browsers that don't have it by default, by placing the following in the parent:

Note, that this will likely prompt the user for confirmation before closing the window.

Q: How can I close a popup window after, say, 5 seconds?
A:

...

Q: How do you return the results from a query into a new window?
A:

Q: Is it possible to disallow a pop-up window to be maximized?
A:

When opening a window use the attribute resizable:

window.open('apage.html','myWIndow','resizable=no');

The code does not work in Internet Explorer - you can maximise the window, and the resizable=no is ignored once you restore the size.

Q: Can i get Javascript counter?
A:

JavaScript CAN NOT do a Web page counter alone. You will have to use a CGI script, or Java as well. The reason for this is that JavaScript can not write to external files, something necessary to run a counter (updating hit count).

Q: How do i clear the user's cache?
A:
There are several things that can't be done with JavaScript for security reasons, including such things as changing user preferences or clearing the cache.

However, I do have a slick workaround that prevents a page from being stored in the user's cache. (You don't have to clear the cache if the page isn't in the cache!) Anyways, on with it. The code to stick in the HEAD section of your page is this:

Note: The second 'Expires' tag is not necessary for all browsers, but some browsers do use with the 'no-cache' tag, so it is best to just leave it in for good measure.

Event Handlers

2008-08-06T07:19:00.000-07:00

JavaScript programs are typically event-driven. Events are actions that occur on the Web page, usually as a result of something the user does, although not always. For example, a button click is an event, as is giving focus to a form element; resizing the page is an event, as is submitting a form. It is these events which cause JavaScript programs to spring into action. For example, if you move your mouse over this phrase, a message will pop-up, courtesy of JavaScript.

An event, then, is the action which triggers an event handler. The event handler specifies which JavaScript code to execute. Often, event handlers are placed within the HTML tag which creates the object on which the event acts:

For example, a hyperlink is subject to a MouseOver event, meaning that its event handler will be triggered when the mouse passes over the link. Therefore, you place the event handler for a hyperlink's MouseOver inside the A tag:

The JavaScript which is called by the event handler may be any valid JavaScript code: a single statement or a series of statements, although most often it is a function call.

The set of all events which may occur, and the particular page elements on which they can occur, is part of the Document Object Model (DOM), and not JavaScript itself (see the earlier section "Document Object Model"). As a result, Netscape and Microsoft do not share the exact same set of events, nor are all page elements subject to the same events between browsers. For example, Internet Explorer 4 supports a MouseOver event for an image while Navigator 4 does not.

The table below illustrates some of the most commonly used events supported in both DOM's. Because the DOM's differ in their event support, the following documents are recommended as an overview of each browser's event support:

Navigator 4: Event Handlers Summary
MSIE 4: DHTML Event Summary

Common Events Event

Occurs when...

Event Handler

click User clicks on form element or link onClick
change User changes value of text, textarea, or select element onChange
focus User gives form element input focus onFocus
blur User removes input focus from form element onBlur
mouseover User moves mouse pointer over a link or anchor onMouseOver
mouseout User moves mouse pointer off of link or anchor onMouseOut
select User selects form element's input field onSelect
submit User submits a form onSubmit
resize User resizes the browser window onResize
load User loads the page in the Navigator onLoad
unload User exits the page onUnload

Objects

2008-08-06T07:18:00.002-07:00

An object is a "package" of data; a collection of properties (variables) and methods (functions) all classed under a single name. For example, imagine that there was an object named car. We could say that the car object possesses several properties: make, model, year, and color, for example. We might even say that car possesses some methods: go(), stop(), and reverse(). Although car is obviously fictional, you can see that its properties and methods all relate to a common theme.

In JavaScript you may create your own objects for storing data. More commonly, though, you will use the many "built-in" objects which allow you to work with, manipulate, and access the Web page and Web browser. This set of pre-existing objects is known as the "Document Object Model".

Document Object Model
Often referred to as the DOM, this object model is a hierarchy of all objects "built in" to JavaScript. Most of these objects are directly related to characteristics of the Web page or browser. The reason we qualify the term "built in" is because the DOM is technically separate from JavaScript itself. That is, the JavaScript language specification, standardized by the ECMA, does not actually specify the nature or specifics of the DOM. Consequently, Netscape and Microsoft have developed their own individual DOM's which are not entirely compatible. Additionally, the DOM stands apart from JavaScript because it could theoretically be accessed by other scripting languages as well.

In summary, then, what we refer to as "JavaScript" is actually made up of both JavaScript, the language, and the DOM, or object model which JavaScript can access. In a future WDVL article we will take a closer look at the DOM and its current and future role.

Below is a graphical chart illustrating a high-level view of Netscape's DOM. Microsoft's DOM is actually a superset of Netscape's, and so the chart below actually represents a subset of Microsoft's own DOM.

Reprinted from Netscape's JavaScript Guide

Properties
Access the properties of an object with a simple notation: objectName.propertyName. Both the object name and property name are case sensitive, so watch your typing. Because a property is essentially a variable, you can create new properties by simply assigning it a value. Assuming, for instance, that carObj already exists (we'll learn to create a new object shortly), you can give it properties named make, model, and year as follows:

carObj.make="Toyota";
carObj.model="Camry";
carObj.year=1990;
document.write(carObj.year);

A JavaScript object, basically, is an array. If you're familiar with other languages you probably recognize an array as a collection of values residing within a single named data structure. You can access an object's properties either using the objectName.propertyName syntax illustrated above, or by using an array syntax:

carObj["make"]="Toyota";
carObj["model"]="Camry";
document.write(carObj["year"]);

Methods
Unlike a basic data array, an object can also contain functions, which are known as methods when part of an object. You call a method using the basic syntax: objectName.methodName(). Any arguments required for the method are passed between the parentheses, just like a normal function call.

For example, the window object possesses a method named close(), which simply closes the specified browser window:

window.close();

Creating Objects
Most of the time you will be referencing objects which are built-in to the DOM. However, you may want to create your own objects for storing data within a JavaScript program. There are several ways to create a new object, but we'll look at two: creating a direct instance of an object and creating an object prototype.

direct instance of an object
Despite the awkward sound name, a "direct instance of an object" simply means creating a new single object, such as myPetDog:

myPetDog=new Object();
myPetDog.name="Barney";
myPetDog.breed="beagle";
myPetDog.year=1981;

Assigning a method to your new object is also simple. Assume that you already have coded a function named woof(), which causes a barking sound to play:

myPetDog.woof=woof;

prototype of an object
Sometimes, you'll want to create a "template" or prototype of an object. This does not create an actual instance of the object, but defines the structure of the object. In the future, then, you can quickly stamp out a particular instance of the object. Suppose that instead of myPetDog, you created a prototype object named petDog. This object could then be a template for a particular pet dog object. First, create a function which defines the petDog structure:

function petDog(name, breed, year)
{ this.name = name;
this.breed = breed;
this.year = year;
}

Now that the petDog prototype has been set, you can quickly create single instances of a new object based on the petDog structure:

myPetDog=new petDog("barney","beagle",1981);
yourPetDog=new petDog("max","terrier",1990);

Functions

2008-08-06T07:18:00.001-07:00

A function groups together a set of statements under a named subroutine. This allows you to conveniently "call" the function whenever its action is required. Functions are a fundamental building block of most JavaScript programs, so you'll become quite familiar with their use. Before you can call on a function, of course, you must first create it. We can break down the use of functions, then, into two logical categories: defining functions and calling functions.

defining functions
The function definition is a statement which describes the function: its name, any values (known as "arguments") which it accepts incoming, and the statements of which the function is comprised.

function funcName(argument1,argument2,etc)
{ statements; }

A function doesn't necessarily require arguments, in which case you need only write out the parenthesis; e.g. funcName(). If you do specify arguments, those arguments will be variables within the function body (the statements which make up the function). The initial values of those variables will be any values passed on by the function call.

Generally it's best to define the functions for a page in the HEAD portion of a document. Since the HEAD is loaded first, this guarantees that functions are loaded before the user has a chance to do anything that might call a function. Alternately, some programmers place all of their functions into a separate file, and include them in a page using the SRC attribute of the SCRIPT tag. Either way, the key is to load the function definitions before any code is executed.

Consider, for example, a simple function which outputs an argument to the Web page, as a bold and blinking message:

function boldblink(message)
{ document.write(""+message+""); }

Some functions may return a value to the calling expression. The following function accepts two arguments, x and y, and returns the result of x raised to the y power:

function raiseP(x,y)
{ total=1;
for (j=0; j { total*=x; }
return total; //result of x raised to y power
}

calling functions
A function waits in the wings until it is called onto the stage. You call a function simply by specifying its name followed by a parenthetical list of arguments, if any:

clearPage();
boldblink("Call me gaudy!");

Functions which return a result should be called from within an expression:

total=raiseP(2,8);
if (raiseP(tax,2)<100) ...

Quite commonly, JavaScript functions are called from within event handlers, which we'll take a look at later in this article.

Statements

2008-08-06T07:17:00.001-07:00

Statements define the flow of a script, known as "program flow." A statement, like a fully grammatical English sentence, is made up of smaller expressions which, altogether, evaluate into a cogent meaning. In JavaScript, statements are organized as either conditionals, loops, object manipulations, and comments.

Good practice suggests that each JavaScript statements should be terminated with a semicolon (;). This is often not strictly necessary, as a new line also serves to separate statements, but when multiple statements reside on the same line the semicolon delimiter is mandatory.

A set of statements that is surrounded by braces is called a block. Blocks of statements are used, for example, in functions and conditionals.

Normally statements are executed sequentially: x = 1; y = 2; z = x + y; but this can be altered by some statements which test a condition and branch or loop according to the result.

Conditionals
Conditional statements direct program flow in specified directions depending upon the outcomes of specified conditions. These tests are a major influence on the order of execution in a program.

if...else
As seen in many programming languages, if the condition evaluates to true then the block of statements1 is executed. Optionally, an else clause specifies a block of statements2 which are executed otherwise. You may omit the else clause if there are no statements which need to be executed if the condition is false.

if (condition)
{ statements1; }

else
{ statements2; }

switch (Netscape & MSIE 4)
Commonly known as a "case statement," switch matches an expression with a specified case, and executes the statements defined for that case. In essence, the switch statement is a sort of shorthand for combining many implied if statements together.

switch (expression){
case label :
statement;
break;
case label :
statement;
break;
...
default : statement;
}

For example, imagine that you wanted to execute different sets of statements depending on whether favoritePet was "dog," "cat," or "iguana." Note that the break; statement prevents any cases below the match from being executed. The default case is matched if none of the cases match the expression.

switch (favoritePet){
case "dog" :
statements;
break;
case "cat" :
statements;
break;
case "iguana" :
statements;
break;
default : statements;
}

Loops
for
The venerable for loop repeatedly cycles through a block of statements until a test condition is false. Typically, the number of times a loop is repeated depends on a counter. The JavaScript for syntax incorporates the counter and its increments:

for (initial-statement; test; increment)
{ statements; }

The initial-statement is executed first, and once only. Commonly, this statement is used to initialize a counter variable. Then the test is applied and if it succeeds then the statements are executed. The increment is applied to the counter variable and then the loop starts again. For instance, consider a loop which executes 10 times:

for (i=0; i<10; i++)
{ statements; }

do...while (Netscape & MSIE 4)
Another loop, a do...while statement executes a block of statements repeatedly until a condition becomes false. Due to its structure, this loop necessarily executes the statement at least once.

do
{ statements;}
while (condition)

while
In similar fashion as the do...while statement, the while statement executes its statement block as long as the condition is true. The main difference between while and do...while, aside from the fact that only while is supported in all JavaScript versions, is that a while loop may not execute the statements even once if the condition is initially false.

while (condition)
{ statements; }

break and continue
Both of these statements may be used to "jump the tracks" of an iterating loop. When used within the statement block of a loop, each statement behaves slightly differently:

break Aborts execution of the loop, drops out of loop to the next statement following the loop.
continue Aborts this single iteration of the loop, returns execution to the loop control, meaning the condition specified by the loop statement. Loop may execute again if condition is still true.

Object manipulation
for...in
The sometimes confusing for...in statement is used to cycle through each property of an object or each element of an array. The idea is that you may want to execute a statement block which operates on every property or element.

for (variable in object)
{ statements; }

Imagine, for example, that an object named wine1 has five properties: vineyard, year, varietal, alcohol, and color. You want to output the value of each property, as if producing a record from a database.

var record = "Wine 1

"
for (var prop in wine1)
{record += prop + " = " + wine1[prop] + "
"}
record += "
"
document.write(record)

with
The with statement serves as a sort of shorthand, allowing you to execute a series of statement who all assume a specified object as the reference. In other words, the object specified in the with statement is used as the default object whenever a property is encountered with no object specified.

with (object)
{ statements; }

Comments
Despite the fact that comments are purely optional, they can easily be a crucial part of your program. Comments can explain the action, like a color commentary, which can be a great help in understanding the code. Whether as a teaching tool or to simply remind yourself what the code does, comments are best sprinkled liberally throughout a program. Remember, comments are for humans, so write them that way!

Comments can also be used for debugging -- you can comment "out" sections of code to prevent them from being executed. In doing so you may learn more about why a certain problem is occurring in your program.

Because JavaScript must ignore comments, there is an appropriate syntax for demarcating text as a comment. For single line comments, simply precede the line with two backslashes. For multi-line comment blocks, begin the comment with /* and close with */.

//A lonely ol' single line comment
/* A dense thicket of commentary, spanning many captivating lines
of explanation and intrigue. */

Operators

2008-08-06T07:16:00.000-07:00

Operators take one or more variables or values (operands) and return a new value; e.g. the '+' operator can add two numbers to produce a third. You use operators in expressions to relate values, whether to perform arithmetic or compare quantities. Operators are divided into several classes depending on the relation they perform:

arithmetic or computational
Arithmetic operators take numerical values (either literals or variables) as their operands and return a single numerical value. The standard arithmetic operators are:

+ Addition
- Subtraction
* Multiplication
/ Division
% Modulus: the remainder after division;
e.g. 10 % 3 yields 1.
++ Unary increment: this operator only takes one operand. The operand's value is increased by 1. The value returned depends on whether the ++ operator is placed before or after the operand; e.g. ++x will return the value of x following the increment whereas x++ will return the value of x prior to the increment.
-- Unary decrement: this operator only takes one operand. The operand's value is decreased by 1. The value returned depends on whether the -- operator is placed before or after the operand; e.g. --x will return the value of x following the decrement whereas x-- will return the value of x prior to the decrement.
- Unary negation: returns the negation of operand.

comparison
A comparison operator compares its operands and returns a logical value based on whether the comparison is true or not. The operands can be numerical or string values. When used on string values, the comparisons are based on the standard lexicographical (alphabetic) ordering.

== "Equal to" returns true if operands are equal.
!= "Not equal to" returns true if operands are not equal.
> "Greater than" returns true if left operand is greater than right operand.
>= "Greater than or equal to" returns true if left operand is greater than or equal to right operand.
< "Less than" returns true if left operand is less than right operand.
<= "Less than or equal to" returns true if left operand is less than or equal to right operand.

boolean
Boolean operators are typically used to combine multiple comparisons into a conditional expression. For example, you might want to test whether (total>100) AND (stateTax=true). A boolean operator takes two operands, each of which is a true or false value, and returns a true or false result.

&& "And" returns true if both operands are true.
|| "Or" returns true if either operand is true.
! "Not" returns true if the negation of the operand is true (e.g. the operand is false).

string
Strings can be compared using the comparison operators. Additionally, you can concatenate strings using the + operator.

"dog" + "bert"
yields
"dogbert"

assignment
The assignment operator (=) lets you assign a value to a variable. You can assign any value to a variable, including another variable (whose value will be assigned). Several shorthand assignment operators allow you to perform an operation and assign its result to a variable in one step.

= Assigns the value of the righthand operand to the variable on the left.
Example: total=100;
Example: total=(price+tax+shipping)
+=
(also -=, *=, /=) Adds the value of the righthand operand to the lefthand variable.
Example: total+=shipping (adds value of shipping to total and assigned result to total)
&=
(also |=) Assigns result of (lefthand operand && righthand operand) to lefthand operand.

special
Several JavaScript operators, rarely used, fall into no particular category. These operators are summarized below.

Conditional operator

(condition) ? trueVal : falseVal Assigns a specified value to a variable if a condition is true, otherwise assigns an alternate value if condition is false.
Example:
preferredPet = (cats > dogs) ? "felines" : "canines"

If (cats>dogs), preferredPet will be assigned the string value "felines," otherwise it will be assigned "canines".
typeof operand Returns the data type of operand.
Example -- test a variable to determine if it contains a number:
if (typeof total=="number") ...

regular expressions (Netscape & MSIE 4)
New to JavaScript 1.2 is support for regular expressions, which are defined patterns used to match character combinations appearing in string values. Regular expressions are very powerful, potentially allowing you to search for any conceivable character pattern. However, they can also be quite complex to construct. Because regular expressions are widely supported in all high-level development environments, it is advised that you consider learning about regular expressions as a subject unto itself.

Two detailed explanations of regular expressions can be found at

Netscape's JavaScript Guide: Chapter 9 Expressions and Operators
PERL5 Regular Expression Description

Variables and Data Types

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Variables store and retrieve data, also known as "values". A variable can refer to a value which changes or is changed. Variables are referred to by name, although the name you give them must conform to certain rules. A JavaScript identifier, or name, must start with a letter or underscore ("_"); subsequent characters can also be digits (0-9). Because JavaScript is case sensitive, letters include the characters "A" through "Z" (uppercase) and the characters "a" through "z" (lowercase). Typically, variable names are chosen to be meaningful regarding the value they hold. For example, a good variable name for containing the total price of goods orders would be total.

scope

When you assign a new variable to an initial value, you must consider the issue of scope. A variable may be scoped as either global or local. A global variable may be accessed from any JavaScript on the page. A local variable may only be accessed from within the function in which it was assigned.

Commonly, you create a new global variable by simply assigning it a value:

newVariable=5;

However, if you are coding within a function and you want to create a local variable which only scopes within that function you must declare the new variable using the var statement:

function newFunction()
{ var loop=1;
total=0;
...additional statements...
}

In the example above, the variable loop will be local to newFunction(), while total will be global to the entire page.

type

A value, the data assigned to a variable, may consist of any sort of data. However, JavaScript considers data to fall into several possible types. Depending on the type of data, certain operations may or may not be able to be performed on the values. For example, you cannot arithmetically multiply two string values. Variables can be these types:

Numbers
3 or 7.987, Integer and floating-point numbers.

Integers can be positive, 0, or negative; Integers can be expressed in decimal (base 10), hexadecimal (base 16), and octal (base 8). A decimal integer literal consists of a sequence of digits without a leading 0 (zero). A leading 0 (zero) on an integer literal indicates it is in octal; a leading 0x (or 0X) indicates hexadecimal. Hexadecimal integers can include digits (0-9) and the letters a-f and A-F. Octal integers can include only the digits 0-7.
A floating-point number can contain either a decimal point, an "e" (uppercase or lowercase), which is used to represent "ten to the power of" in scientific notation, or both. The exponent part is an "e" or "E" followed by an integer, which can be signed (preceded by "+" or "-"). A floating-point literal must have at least one digit and either a decimal point or "e" (or "E").

Booleans
True or False. The possible Boolean values are true and false. These are special values, and are not usable as 1 and 0. In a comparison, any expression that evaluates to 0 is taken to be false, and any statement that evaluates to a number other than 0 is taken to be true.
Strings
"Hello World !" Strings are delineated by single or double quotation marks. (Use single quotes to type strings that contain quotation marks.)
Objects
myObj = new Object();
Null
Not the same as zero - no value at all. A null value is one that has no value and means nothing.
Undefined
A value that is undefined is a value held by a variable after it has been created, but before a value has been assigned to it.

That said, JavaScript is a loosely typed language -- you do not have to specify the data type of a variable when you declare it, and data types are converted automatically as needed during script execution. By and large, you may simply assign any type of data to any variable. The only time data typing matters is when you need to perform operations on the data. Certain operators behave differently depending on the type of data being deal with. For example, consider the + operator:

"5" + "10"
yields
"510" (string concatenation)

5 + 10
yields
15 (arithmetic sum)

JavaScript Grammar

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JavaScript code, much like other programming languages, is made up of statements which serve to make assignments, compare values, and execute other sections of code. By and large, programmers will already be familiar with JavaScript's usage of variables, operators, and statements. Below is a chart summarizing the main elements of JavaScript grammar. Following, we will look at each element in detail.

Variables
Labels which refer to a changeable value.
Example: total may be possess a value of 100.
Operators
Actors which can be used to calculate or compare values.
Example: Two values may be summed using the addition operator (+); total+tax
Example: Two values may be compared using the greater-than operator (>); total>200
Expressions
Any combination of variables, operators, and statements which evaluate to some result. In English parlance this might be termed a "sentence" or even a "phrase", in that grammatical elements are combined into a cogent meaning.
Example: total=100;
Example: if (total>100)
Statements
As in English, a statement pulls all grammatical elements together into a full thought. JavaScript statements may take the form of conditionals, loops, or object manipulations. It is good form to separate statements by semicolons, although this is only mandatory if multiple statements reside on the same line.
Example: if (total>100) {statements;} else {statements;}
Example: while (clicks<10) {statements;}
Objects
Containing constructs which possess a set of values, each value reflected into an individual property of that object. Objects are a critical concept and feature of JavaScript. A single object may contain many properties, each property which acts like a variable reflecting a certain value. JavaScript can reference a large number of "built-in" objects which refer to characteristics of a Web document. For instance, the document object contains properties which reflect the background color of the current document, its title, and many more. For a fuller explanation of the built-in objects of JavaScript, see the section on "Document Object Model".
Functions and Methods
A JavaScript function is quite similar to a "procedure" or "subroutine" in other programming languages. A function is a discrete set of statements which perform some action. It may accept incoming values (parameters), and it may return an outgoing value. A function is "called" from a JavaScript statement to perform its duty. A method is simply a function which is contained in an object. For instance, a function which closes the current window, named close(), is part of the window object; thus, window.close() is known as a method.

JavaScript Tutorial for Programmers

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JavaScript code is typically embedded into an HTML document using the SCRIPT tag. You are free to embed as many scripts into a single document as you like, using multiple SCRIPT tags. A script embedded in HTML with the SCRIPT tag uses the format:

The LANGUAGE attribute is optional, but recommended. You may specify that a section of code only be executed by browsers which support a particular version of JavaScript; for instance:

The external file is simply a text file containing JavaScript code, and whose filename ends with the extension ".js". Note that although some version 3 browsers support the SRC attribute, it only functions reliably across platforms in the version 4 browsers.

Scripts can be placed inside comment fields to ensure that your JavaScript code is not displayed by old browsers that do not recognize JavaScript. The markup to begin a comment field is . This practice is certainly optional, but considered good form when your page is likely to be visited by older browsers. Certainly, as older browsers fade away, this practice will likely become unnecessary.

Java Script Faq's &

JavaScript Tutorial

What You Should Already Know

What is JavaScript?

Are Java and JavaScript the Same?

What can a JavaScript Do?

" + name + "

Example Explained

Examples

Where to Put the JavaScript

Using an External JavaScript

Examples

Variables

Declare a Variable

Assign a Value to a Variable

Lifetime of Variables

Examples

Conditional Statements

If Statement

Syntax

Example 1

Example 2

If...else Statement

Syntax

Example

If...else if...else Statement

Syntax

Example

Examples

The JavaScript Switch Statement

Syntax

Example

Arithmetic Operators

Assignment Operators

Comparison Operators

Logical Operators

String Operator

Conditional Operator

Syntax

Example

Examples

Alert Box

Confirm Box

Prompt Box

Examples

JavaScript Functions

Example

How to Define a Function

The return Statement

Example

Examples

JavaScript Loops

The for Loop

The while loop

Examples

The while loop

The do...while Loop

Examples

JavaScript break and continue Statements

Break

Continue

Examples

JavaScript For...In Statement

Example

Events

onload and onUnload

onFocus, onBlur and onChange

onSubmit

onMouseOver and onMouseOut

Examples

JavaScript - Catching Errors

Try...Catch Statement

Syntax

Example 1

Example 2

The onerror Event

Examples

The Throw Statement

Syntax

Example 1