Elements

txt2="nice day"; txt3=txt1+txt2;

After the execution of the statements above, the variable txt3 contains "What a verynice day".

or insert a space into the expression: txt1="What a very"; txt2="nice day"; txt3=txt1+" "+txt2;

"What a very nice day"

3)
COMPARISON OPERATORS:

Comparison operators are used in logical statements to determine equality or difference between variables or values. Given that x=5
, the table below explains the comparison operators:

 Operator
 
Description  
Example

 ==  is equal to  x==8 is false

 ===  is exactly equal to (value and type)  x===5 is true

x==="5" is false

 !=  is not equal  x!=8 is true

 >  is greater than  x>8 is false

 <  is less than  x<8 is true

 >=  is greater than or equal to  x>=8 is false

 <=  is less than or equal to  x<=8 is true

LOGICAL OPERATORS:

Logical operators are used to determine the logic between variables or values. Given that x=6 and y=3.

CONDITIONAL OPERATORS:

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
has the value of "PRES", then the variable greeting
will be assigned the value "Dear President" else it will be assigned "Dear".

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:

i) if statement
- use this statement to execute some code only if a specified condition is true ii) if...else statement
- use this statement 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 to select one of many blocks of code to be executed

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

i
)
IF STATEMENT

Syntax:

if (condition)

{

code to be executed if condition is true }

Example:

<script type="text/javascript"> //Write a "Good morning" greeting if

//the time is less than 10 var d=new Date(); var time=d.getHours(); if (time<10)

{

document.write("<b>Good morning</b>");

}

</script>

ii)
IF...ELSE STATEMENT

Use the If...else statement to execute some code if a condition is true and another code if the condition is not true. Syntax:

if (condition)

{

code to be executed if condition is true

} else

{

code to be executed if condition is not true }

Example:

<script type="text/javascript">

//If the time is less than 10, you will get a "Good morning" greeting.

//Otherwise you will get a "Good day" greeting. var d = new Date(); var time = d.getHours(); if (time < 10)

{

document.write("Good morning!");

} else

{

document.write("Good day!");

}

</script>

iii)
IF...ELSE IF...ELSE STATEMENT:

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:

<script type="text/javascript"> var d = new Date() var time = d.getHours() if (time<10)

document.write("<b>Good morning</b>"); else if (time>10 && time<16)

document.write("<b>Good day</b>"); else

document.write("<b>Hello World!</b>");

</script>

iv)
SWITCH STATEMENT:

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 }

Example:

<script type="text/javascript">

//You will receive a different greeting based //on what day it is. Note that Sunday=0, //Monday=1, Tuesday=2, etc. var d=new Date(); theDay=d.getDay(); switch (theDay)

{

case 5: document.write("Finally Friday");

   break;

case 6: document.write("Super Saturday");

   break;

case 0: document.write("Sleepy Sunday");  break;

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

}

</script>

JAVASCRIPT POPUP BOXES:

JavaScript has three kinds of popup boxes:

i) Alert Box ii) Confirm Box or Message Box, and iii) Prompt Box.

i
)
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"); Example:
<!DOCTYPE html> <html>

<head>

<script type="text/javascript"> function show_alert()

{

alert("You are Clicked Me ");

}

</script>

</head>

<body>

<input type="button" onClick="show_alert()" value="Click Me " />

</body> </html>

ii)
CONFIRM BOX OR MESSAGE BOX:

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

Example:

<!DOCTYPE html> <!DOCTYPE html> <!DOCTYPE html>

<head>

<script type="text/javascript"> function show_confirm()

{

var r=confirm("Press a button"); if (r==true)

{

alert("You pressed OK!");

} else

{

alert("You pressed Cancel!");

}

}

</script>

</head>

<body>

<input type="submit" onClick="show_confirm()" value="Clicked Me " />

</body> </html> iii)
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");

Example:

<!DOCTYPE html>

<head>

<script type="text/javascript"> function show_prompt()

{

var name=prompt("Please enter your name","Harry Potter"); if (name!=null && name!="")

{

document.write("Hello " + name + "! How are you today?");

}

}

</script>

</head>

<body>

<input type="button" onclick="show_prompt()" value="Show prompt box" /> </body>

</html>

JAVASCRIPT FUNCTIONS

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

How to Define a Function Syntax:

function function-name(var1,var2,...,varX)

{ some code; }

The parameters var1, var2, etc. are variables or values passed into the function. The { and the } defines the start and end of the function.

Note:
A function with no parameters must include the parentheses () after the functio name.
Note:
Do not forget about the importance of capitals in JavaScript! The word function must be written in lowercase letters, otherwise a JavaScript error occurs! Also note that you must call a function with the exact same capitals as in the function name.

Example:

<!DOCTYPE html>

<head>

<script type="text/javascript"> function displaymessage()

{

alert("Hello World!");

}

</script>

</head>

<body>

<form>

<input type="button" value="Click me" onClick="displaymessage()" /> </form>

</body>

</html>

If the line: alert("Hello world!!") in the example above had not been put within a function, it would have been executed as soon as the page was loaded. Now, the script is not executed before a user hits the input button. The function displaymessage() will be executed if the input button is clicked.

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. The example below returns the product of two numbers (a and b):

Example:

<!DOCTYPE html>

<head>

<script type="text/javascript"> function product(a,b)

{

return a*b; }

</script>

</head>

<body>

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

</script>

</body> </html>

THE LIFETIME OF JAVASCRIPT VARIABLES:

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

JAVASCRIPT LOOPS

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 kinds of loops:

i)  for
- loops through a block of code a specified number of times ii) while
- loops through a block of code while a specified condition is true

i
)
FOR LOOP

Syntax:

for (var=startvalue;var<=endvalue;var=var+increment)

{

code to be executed

}

Example:

The example below defines a loop that starts with i=0. The loop will continue to run as long as i
is less than, or equal to 5. 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.

<!DOCTYPE html>

<body>

<script type="text/javascript"> var i=0;

for (i=0;i<=5;i++)

{

document.write("The number is " + i); document.write("<br />");

}

</script>

</body> </html>

ii)
WHILE LOOP

The while loop loops through a block of code while a specified condition is true.

Syntax:
while (var<=endvalue)

{

code to be executed

}

Note:
The <= could be any comparing operator.

Example:

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 5. i
will increase by 1 each time the loop runs:

<html>

<body>

<script type="text/javascript"> var i=0; while (i<=5)

{

document.write("The number is " + i); document.write("<br />");

i++;

}

</script> </body></html>

iii)
DO...WHILE LOOP

The do...while loop is a variant of the while loop. This loop will execute the block of code ONCE, and then it will repeat the loop as long as the specified condition is true.

Syntax:

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

Example:

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

<html>

<body>

<script type="text/javascript"> var i=0; do

{

document.write("The number is " + i); document.write("<br />");

i++;

}

while (i<=5); </script>

</body> </html>

iv)
THE BREAK STATEMENT

The break statement will break the loop and continue executing the code that follows after the loop (if any).

Example:

<html>

<body> <script type="text/javascript"> var i=0;

for (i=0;i<=10;i++)

{ if (i==3)

break;

document.write("The number is " + i); document.write("<br />");

}

</script>

</body></html>

v)
THE CONTINUE STATEMENT

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

Example:

<html>

<body>

<script type="text/javascript"> var i=0

for (i=0;i<=10;i++)

{ if (i==3)

{ continue;

}

document.write("The number is " + i); document.write("<br />");

}

</script>

</body>

</html> vi)
JAVASCRIPT FOR...IN STATEMENT

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

Syntax:

for (variable in object)

{

code to be executed

}

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

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

Example:

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

<html>

<body>

<script type="text/javascript"> var x;

var mycars = new Array(); mycars[0] = "Saab"; mycars[1] = "Volvo"; mycars[2] = "BMW";

for (x in mycars)

{

document.write(mycars[x] + "<br />");

}

</script>

</body>

</html>

JAVASCRIPT EVENTS:

By using JavaScript, we have the ability to create dynamic web pages. Events are actions that can be detected by JavaScript. Every element on a web page has certain events which can trigger a JavaScript. For example, we can use the onClick event of a button element to indicate that a function will run when a user clicks on the button. We define the events in the HTML tags.

Examples of events:

A mouse click

A web page or an image loading

Mousing over a hot spot on the web page

Selecting an input field in an HTML form

Submitting an HTML form

A keystroke

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

ONLOAD AND ONUNLOAD:

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

ONFOCUS, ONBLUR AND ONCHANGE:

The onFocus, onBlur and onChange events are often used in combination with validation of form fields. Below is an example of how to use the onChange event. The checkEmail() function will be called whenever the user changes the content of the field:

<input type="text" size="30" onChange="checkEmail()">

ONSUBMIT:

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

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

ONMOUSEOVER AND ONMOUSEOUT:

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

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

JAVASCRIPT 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!

Examples:

The example below is supposed to alert "Welcome guest!" when the button is clicked. However, there's a typo in the message() function. alert() is misspelled as adddlert(). A JavaScript error occurs. The catch block catches the error and executes a custom code to handle it. The code displays a custom error message informing the user what happened:

<html>

<head>

<script type="text/javascript"> var txt=""; function message()

{ try

{

adddlert("Welcome guest!");

}

catch(err)

{

txt="There was an error on this page.\n\n"; txt+="Error description: " + err.description + "\n\n"; txt+="Click OK to continue.\n\n"; alert(txt);

}

}

</script>

</head>

<body>

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

</body>

</html>

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:

Example:

<html>

<head>

<script type="text/javascript"> var txt=""; function message()

{

try {

adddlert("Welcome guest!");

} catch(err) { txt="There was an error on this page.\n\n"; txt+="Click OK to continue viewing this page,\n"; txt+="or Cancel to return to the home page.\n\n"; if(!confirm(txt))

{

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

}

}

}

</script>

</head>

<body>

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

</body> </html>

THE THROW STATEMENT

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

Syntax

Throw(exception)

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

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

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

JAVASCRIPT SPECIAL CHARACTERS

The backslash (\) is used to insert apostrophes, new lines, quotes, and other special characters into a text string.

Look at the following JavaScript code:

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

 In JavaScript, a string is started and stopped with either single or double quotes. This means that the string above will be chopped to: We are the so-called

To solve this problem, you must place a backslash (\) before each double quote in "Viking". This turns each double quote into a string literal:

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

JavaScript will now output the proper text string: We are the so-called "Vikings" from the north. Here is another example: document.write ("You \& I are singing!");

The example above will produce the following output:

You & I are singing!

The table below lists other special characters that can be added to a text string with the backslash sign: