Monday, January 23, 2012

Java Script Basics

What is JavaScript ?

JavaScript is:
  • JavaScript is a lightweight, interpreted programming language
  • Designed for creating network-centric applications
  • Complementary to and integrated with Java
  • Complementary to and integrated with HTML
  • Open and cross-platform

JavaScript Syntax:

A JavaScript consists of JavaScript statements that are placed within the <script>... </script> HTML tags in a web page.
You can place the <script> tag containing your JavaScript anywhere within you web page but it is preferred way to keep it within the <head> tags.
The <script> tag alert the browser program to begin interpreting all the text between these tags as a script. So simple syntax of your JavaScript will be as follows
<script ...>
  JavaScript code
</script>

The script tag takes two important attributes:
  • language: This attribute specifies what scripting language you are using. Typically, its value will be javascript. Although recent versions of HTML (and XHTML, its successor) have phased out the use of this attribute.
  • type: This attribute is what is now recommended to indicate the scripting language in use and its value should be set to "text/javascript".
So your JavaScript segment will look like:
<script language="javascript" type="text/javascript">
  JavaScript code
</script>

Your First JavaScript Script:

Let us write our class example to print out "Hello World".
<html>
<body>
<script language="javascript" type="text/javascript">
<!--
   document.write("Hello World!")
//-->
</script>
</body>
</html>
Above code will display following result:
Hello World!

Whitespace and Line Breaks:

JavaScript ignores spaces, tabs, and newlines that appear in JavaScript programs.
Because you can use spaces, tabs, and newlines freely in your program so you are free to format and indent your programs in a neat and consistent way that makes the code easy to read and understand.

Semicolons are Optional:

Simple statements in JavaScript are generally followed by a semicolon character, just as they are in C, C++, and Java. JavaScript, however, allows you to omit this semicolon if your statements are each placed on a separate line. For example, the following code could be written without semicolons
<script language="javascript" type="text/javascript">
<!--
  var1 = 10
  var2 = 20
//-->
</script>
But when formatted in a single line as follows, the semicolons are required:
<script language="javascript" type="text/javascript">
<!--
  var1 = 10; var2 = 20;
//-->
</script>
Note: It is a good programming practice to use semicolons.

Case Sensitivity:

JavaScript is a case-sensitive language. This means that language keywords, variables, function names, and any other identifiers must always be typed with a consistent capitalization of letters.
So identifiers TimeTIme and TIME will have different meanings in JavaScript.
NOTE: Care should be taken while writing your variable and function names in JavaScript.

Comments in JavaScript:

JavaScript supports both C-style and C++-style comments, Thus:
  • Any text between a // and the end of a line is treated as a comment and is ignored by JavaScript.
  • Any text between the characters /* and */ is treated as a comment. This may span multiple lines.
  • JavaScript also recognizes the HTML comment opening sequence <!--. JavaScript treats this as a single-line comment, just as it does the // comment.
  • The HTML comment closing sequence --> is not recognized by JavaScript so it should be written as //-->.

JavaScript Placement in HTML File:

There is a flexibility given to include JavaScript code anywhere in an HTML document. But there are following most preferred ways to include JavaScript in your HTML file.
  • Script in <head>...</head> section.
  • Script in <body>...</body> section.
  • Script in <body>...</body> and <head>...</head> sections.
  • Script in and external file and then include in <head>...</head> section.

JavaScript DataTypes:

JavaScript allows you to work with three primitive data types:
  • Numbers eg. 123, 120.50 etc.
  • Strings of text e.g. "This text string" etc.
  • Boolean e.g. true or false.
JavaScript also defines two trivial data types, null and undefined, each of which defines only a single value.

JavaScript Variables:

Like many other programming languages, JavaScript has variables. Variables can be thought of as named containers. You can place data into these containers and then refer to the data simply by naming the container.
Before you use a variable in a JavaScript program, you must declare it. Variables are declared with the var keyword as follows:
<script type="text/javascript">
<!--
var money;
var name;
//-->
</script>

JavaScript Variable Scope:

The scope of a variable is the region of your program in which it is defined. JavaScript variable will have only two scopes.
  • Global Variables: A global variable has global scope which means it is defined everywhere in your JavaScript code.
  • Local Variables: A local variable will be visible only within a function where it is defined. Function parameters are always local to that function.

JavaScript Variable Names:

While naming your variables in JavaScript keep following rules in mind.
  • You should not use any of the JavaScript reserved keyword as variable name. These keywords are mentioned in the next section. For example, break or boolean variable names are not valid.
  • JavaScript variable names should not start with a numeral (0-9). They must begin with a letter or the underscore character. For example, 123test is an invalid variable name but_123test is a valid one.
  • JavaScript variable names are case sensitive. For example, Name and name are two different variables.

JavaScript Reserved Words:

The following are reserved words in JavaScript. They cannot be used as JavaScript variables, functions, methods, loop labels, or any object names.
abstract
boolean
break
byte
case
catch
char
class
const
continue
debugger
default
delete
do
double
else
enum
export
extends
false
final
finally
float
for
function
goto
if
implements
import
in
instanceof
int
interface
long
native
new
null
package
private
protected
public
return
short
static
super
switch
synchronized
this
throw
throws
transient
true
try
typeof
var
void
volatile
while
with

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