JavaScript


Introduction to JavaScript

JavaScript DataTypes

Numbers: e.g. 23, 4.3, -230, 4.4e-24 etc
Booleans: e.g. True, False
Strings: e.g. “hello”, “what is the time?”
Undefined

Declaring Variables

var height
var name, address, phoneNumber

Conditional Statements

Conditional statements are used to perform different actions based on different conditions.
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 to execute some code only if a specified condition is true
  • 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

If Statement

Use the if statement 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

<script type="text/javascript">
//Write a "Good morning" greeting if
//the time is less than 10

var d=new Date();  //creates a object “d” using Date constructor
var time=d.getHours(); // calls a getHours() function using “d” object and returns to variable “time”

if (time<10)
  {
  document.write("<b>Good morning</b>");
  }
</script>
Notice that there is no ..else.. in this syntax. You tell the browser to execute some code only if the specified condition is true.

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>



If...else if...else Statement

Use the if....else if...else statement to select one of several blocks of code to be executed.

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>

Switch Statement

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

The JavaScript Switch Statement

Use the switch statement 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

<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();
var 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 
For Loop

Loops execute a block of code a specified number of times, or while a specified condition is true.
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:
  • 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 (variable=startvalue;variable<=endvalue;variable=variable+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.

Example
<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>



While Loop

Loops execute a block of code a specified number of times, or while a specified condition is true.

The while Loop

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

Syntax

while (variable<=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:

Example

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



The 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 (variable<=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:

Example

<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>


JavaScript Functions

A function will be executed by an event or by a call to the function.
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.
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.
Things that are important
·         Name of the function
·         The number and type of parameters the function expects
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 functionname(var1,var2,...,varX)
{
some code
}
The parameters var1, var2, etc. are variables or values passed into the function. The { and the } defines the start and end of the function.
Note: A function with no parameters must include the parentheses () after the function name.
How to Call a Function
Syntax
functionname()
or
functionname(arg1,arg2,...,argN)
{
some code
}
A function with no parameters must include the parentheses () after the function name.
The parameters or arguments or the values those which are passed to the function. The { and the } defines the start and end of the function.
What is an Event ?
JavaScript's interaction with HTML is handled through events that occur when the user or browser manipulates a page.
When the page loads, that is an event. When the user clicks a button, that click, too, is an event. Another example of events are like pressing any key, closing window, resizing window etc.
Onclick Event

JavaScript Function Example
Example
<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.
onmouseover and onmouseout Event
These two event types will help you to create nice effects with images or even with text as well. The onmouseover event occurs when you bring your mouse over any element and the onmouseout occurs when you take your mouse out from that element.
Example:
Following example shows how a division reacts when we bring our mouse in that division:
Example
<html>
<head>
<script type="text/javascript">
<!--
function over() {
   alert("Mouse Over");
}
function out() {
   alert("Mouse Out");
}
//-->
</script>
</head>
<body>
<div onmouseover="over()" onmouseout="out()">
<h2> This is inside the division </h2>
</div>
</body>
</html>



JavaScript Popup Boxes

JavaScript has three kind of popup boxes: Alert box, Confirm box, and 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");

Example
<html>
<head>
<script type="text/javascript">
function show_alert()
{
alert("I am an alert box!");
}
</script>
</head>
<body>

<input type="button" onclick="show_alert()" value="Show alert box" />

</body>
</html>




MessageBox Example
Example
<html>
<head>
<script type="/text/javascript">
function MsgBox (textstring) {
alert (textstring) }
</script>
</head>
<body>
<form>
<input name="text1" type=Text>
<input name="submit" type=Button VALUE="Show Me" onClick="MsgBox(form.text1.value)">
</form>
</body>
</html>



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");

Example
<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="button" onclick="show_confirm()" value="Show confirm box" />

</body>
</html>



Prompt Box or Inputbox
A prompt box or inputbox 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
<html>
<body>
<script type="text/javascript">{
var a=prompt("Firstname: ");
var b=prompt("Lastname: ");
document.write("<br />");
document.write("Fullname: ",a + b);
}
</script>
</body>
</html>

 

Example

<html>
<body>
<script type="text/javascript">
var n = prompt("Enter the value of n : ", "0");
if (n>=1 && n<10)
{
document.write("n is b/w 1 and 10")
}
else
{
document.write("n is something else")
}
</script>
</body>
</html>


 

Example

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

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

</body>
</html>



The Lifetime of JavaScript Variables

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