What is JavaScript?
JavaScript was designed to add interactivity to HTML pages
JavaScript is a scripting language
A scripting language is a lightweight programming language
JavaScript is usually embedded directly into HTML pages
JavaScript is an interpreted language (means that scripts execute without preliminary compilation)
Everyone can use JavaScript without purchasing a license
What can a JavaScript do?
JavaScript gives HTML designers a programming tool - HTML authors are normally not programmers, but JavaScript is a scripting language with a very simple syntax! Almost anyone can put small "snippets" of code into their HTML pages
JavaScript can put dynamic text into an HTML page - A JavaScript statement like this: document.write("
" + name + "
") can write a variable text into an HTML pageJavaScript can react to events - A JavaScript can be set to execute when something happens, like when a page has finished loading or when a user clicks on an HTML element
JavaScript can read and write HTML elements - A JavaScript can read and change the content of an HTML element
JavaScript can be used to validate data - A JavaScript can be used to validate form data before it is submitted to a server. This saves the server from extra processing
JavaScript can be used to detect the visitor's browser - A JavaScript can be used to detect the visitor's browser, and - depending on the browser - load another page specifically designed for that browser
JavaScript can be used to create cookies - A JavaScript can be used to store and retrieve information on the visitor's computer
JavaScript's official name is ECMAScript.
ECMA-262 is the official JavaScript standard.
The language was invented by Brendan Eich at Netscape (with Navigator 2.0), and has appeared in all Netscape and Microsoft browsers since 1996.
The development of ECMA-262 started in 1996, and the first edition of was adopted by the ECMA General Assembly in June 1997.
The standard was approved as an international ISO (ISO/IEC 16262) standard in 1998.
The development of the standard is still in progress.
Comment line:
*-***********************
Put your functions in the head section, this way they are all in one place, and they do not interfere with page content.
If you don't want your script to be placed inside a function, or if your script should write page content, it should be placed in the body section.
If you want to run the same JavaScript on several pages, without having to write the same script on every page, you can write a JavaScript in an external file.
Save the external JavaScript file with a .js file extension.
JavaScript is Case Sensitive
Single line comments start with //.
Multi line comments start with /* and end with */.
Rules for JavaScript variable names:
Variable names are case sensitive (y and Y are two different variables)
Variable names must begin with a letter or the underscore character
A variable's value can change during the execution of a script. You can refer to a variable by its name to display or change its value.
The script above declares a variable,
assigns a value to it, displays the value, changes the value,
and displays the value again.
You can declare JavaScript variables with the var statement:
var x;
var carname;
After the execution of the statements above, the variable x will still have the value of 5. The value of x is not reset (or cleared) when you redeclare it.
As with algebra, you can do arithmetic operations with JavaScript variables:
y=x-5;
z=y+5;
y=5;
Operator
Description
Example
Result
+
Addition
x=y+2
x=7
-
Subtraction
x=y-2
x=3
*
Multiplication
x=y*2
x=10
/
Division
x=y/2
x=2.5
%
Modulus (division remainder)
x=y%2
x=1
++
Increment
x=++y
x=6
--
Decrement
x=--y
x=4
Operator
Example
Same As
Result
=
x=y
x=5
+=
x+=y
x=x+y
x=15
-=
x-=y
x=x-y
x=5
*=
x*=y
x=x*y
x=50
/=
x/=y
x=x/y
x=2
%=
x%=y
x=x%y
x=0
The + Operator Used on Strings
The + operator can also be used to add string variables or text values together.
To add two or more string variables together, use the + operator.
txt1="What a very";
txt2="nice day";
txt3=txt1+txt2;
After the execution of the statements above, the variable txt3 contains "What a verynice day".
Comparison and Logical operators are used to test for true or false.
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
Comparison operators can be used in conditional statements to compare values and take action depending on the result:
if (age<18) document.write("Too young");
Logical Operators
Logical operators are used to determine the logic between variables or values.
Given that x=6 and y=3, the table below explains the logical operators:
Operator
Description
Example
&&
and
(x < 10 && y > 1) is true
||
or
(x==5 || y==5) is false
!
not
!(x==y) is true
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 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:
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
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.
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");
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 for Loop
The while loop
The break Statement
The break statement will break the loop and continue executing the code that follows after the loop (if any).
The continue Statement
The continue statement will break the current loop and continue with the next value.
JavaScript For...In Statement
The for...in statement loops through the elements of an array or through the properties of an object.
Events are actions that can be detected by JavaScript.
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!
Nuvve nenani anukunnane
manase neeke arpinchane
kalalo elalo nee rupam chusane
pade pade nenna chudalani talechane
kani.............
devaltalu eppudu manatho undarani telesinde
okka shanam nennu chuse aavakasam is the
aa okka shanam kosam veye sarlu chavadaniki siddamga unnane
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:
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:
