JAVASCRIPT Notes
JAVASCRIPT
JavaScript is the scripting language of the Web.
JavaScript is used in millions of Web pages to add functionality, validate forms, detect browsers, and
much more.
Introduction to JavaScript
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, and Opera.
What is JavaScript?
JavaScript was designed to add interactivity to HTML pages
JavaScript is a scripting language
A scripting language is a lightweight programming language
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
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("<h1>" + name + "</h1>") 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 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
Page 1
, JAVASCRIPT Notes
JavaScript can be used to create cookies - A JavaScript can be used to store and retrieve
information on the visitor's computer.
JavaScript Variables
Variables are "containers" for storing information.
JavaScript variables are used to hold values or expressions.
A variable can have a short name, like x, or a more descriptive name, like carname.
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
Note: Because JavaScript is case-sensitive, variable names are case-sensitive.
Example
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.
<html>
<body>
<script type="text/javascript">
var firstname;
firstname="Welcome";
document.write(firstname);
document.write("<br />");
firstname="XYZ";
document.write(firstname);
</script>
<p>The script above declares a variable,
assigns a value to it, displays the value, change the value,
and displays the value again.</p>
</body>
</html>
Output :
Welcome
XYZ
The script above declares a variable, assigns a value to it, displays the value, change the value, and
displays the value again.
Page 2
, JAVASCRIPT Notes
Declaring (Creating) JavaScript Variables
Creating variables in JavaScript is most often referred to as "declaring" variables.
You can declare JavaScript variables with the var statement:
var x;
var carname;
After the declaration shown above, the variables are empty (they have no values yet).
However, you can also assign values to the variables when you declare them:
var x=5;
var carname="Scorpio";
After the execution of the statements above, the variable x will hold the value 5, and carname will hold
the value Scorpio.
Note: When you assign a text value to a variable, use quotes around the value.
Assigning Values to Undeclared JavaScript Variables
If you assign values to variables that have not yet been declared, the variables will automatically be
declared.
These statements:
x=5;
carname="Scorpio";
have the same effect as:
var x=5;
var carname="Scorpio";
Redeclaring JavaScript Variables
If you redeclare a JavaScript variable, it will not lose its original value.
var x=5;
var x;
Page 3
, JAVASCRIPT Notes
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.
Page 4
JAVASCRIPT
JavaScript is the scripting language of the Web.
JavaScript is used in millions of Web pages to add functionality, validate forms, detect browsers, and
much more.
Introduction to JavaScript
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, and Opera.
What is JavaScript?
JavaScript was designed to add interactivity to HTML pages
JavaScript is a scripting language
A scripting language is a lightweight programming language
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
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("<h1>" + name + "</h1>") 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 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
Page 1
, JAVASCRIPT Notes
JavaScript can be used to create cookies - A JavaScript can be used to store and retrieve
information on the visitor's computer.
JavaScript Variables
Variables are "containers" for storing information.
JavaScript variables are used to hold values or expressions.
A variable can have a short name, like x, or a more descriptive name, like carname.
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
Note: Because JavaScript is case-sensitive, variable names are case-sensitive.
Example
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.
<html>
<body>
<script type="text/javascript">
var firstname;
firstname="Welcome";
document.write(firstname);
document.write("<br />");
firstname="XYZ";
document.write(firstname);
</script>
<p>The script above declares a variable,
assigns a value to it, displays the value, change the value,
and displays the value again.</p>
</body>
</html>
Output :
Welcome
XYZ
The script above declares a variable, assigns a value to it, displays the value, change the value, and
displays the value again.
Page 2
, JAVASCRIPT Notes
Declaring (Creating) JavaScript Variables
Creating variables in JavaScript is most often referred to as "declaring" variables.
You can declare JavaScript variables with the var statement:
var x;
var carname;
After the declaration shown above, the variables are empty (they have no values yet).
However, you can also assign values to the variables when you declare them:
var x=5;
var carname="Scorpio";
After the execution of the statements above, the variable x will hold the value 5, and carname will hold
the value Scorpio.
Note: When you assign a text value to a variable, use quotes around the value.
Assigning Values to Undeclared JavaScript Variables
If you assign values to variables that have not yet been declared, the variables will automatically be
declared.
These statements:
x=5;
carname="Scorpio";
have the same effect as:
var x=5;
var carname="Scorpio";
Redeclaring JavaScript Variables
If you redeclare a JavaScript variable, it will not lose its original value.
var x=5;
var x;
Page 3
, JAVASCRIPT Notes
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.
Page 4
