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JavaScript Functions

You can embed JavaScript in an HTML document in the following ways:

As statements within certain HTML tags and as functions or statements within a <SCRIPT LANGUAGE="JavaScript"> . . . </SCRIPT> tag set.
Make sure to have

between the SCRIPT tags.
While the SCRIPT tags won't be processed by browsers that don't recognize them, the HTML comment block is needed to "hide" the comment from the non-JavaScript capable browsers. (Otherwise they will just display the JavaScript as text. The JavaScript processors will ignore the HTML comment symbols.)
By specifying a file to be included as the JavaScript source (rather than embedding the JavaScript in the HTML).
By specifying a JavaScript expression as the value for an HTML attribute (in IMG tags).

As event handlers within certain other HTML tags (mostly form elements).
JavaScript statements can either be executed as soon as encountered in your page by the browser, or deferred for later execution, when called as a result of some action. JavaScript functions are in this second group. Normally you want to include JavaScript functions between the <HEAD> and </HEAD> tags on your page. This insures that the functions will be completely loaded before they can be executed by some action specified in the body of the page.
In this section we will concentrate on JavaScript functions.
JavaScript functions bear a strong resemblance to C programming language functions. If you can program in C, you will have little trouble with JavaScript. But there are significant differences.

C Function JavaScript Function

Function format:
function_name (arg_type argument )
{
statement ; statement ;
statement ;
. . . return ;
}
Function format same except:
  ;    is optional, required only if more than one statement on a line
arg_type is not used

function's arguments are typed function's arguments are not typed

Called functions need prototype No function prototypes

Pointers (addresses) can be used No pointers allowed

Variables must be explicitly typed as int, float, char, etc. Variables don't need to be typed, just assigned a value or declared with a var
Variable Names
A JavaScript identifier or name must start with a letter or underscore ("_"); subsequent characters can also include digits (0-9). Letters include the characters "A" through "Z" (uppercase) and the characters "a" through "z" (lowercase). JavaScript is case-sensitive (Abc is not the same variable as abc).
Variable scope
The scope of a variable is where you can reference it in a script. In JavaScript a variable can have either of two scopes:

global: you can use the variable anywhere in the application.
local: you can use the variable only within the current function.
To declare a local variable inside a function, use the var keyword, for example:
var total = 0
To declare a global variable, declare the variable by assignment, that is, simply assign the desired value to the variable (either in a function or outside a function). total = 0

No inherent objects Many predefined objects with methods and properties

Must be compiled before execution No compilation. Each statement is processed by interpreter every time it is used.

All JavaScript statements and functions in a page need to be enclosed in <SCRIPT> tags to flag them to the JavaScript interpreter. You should always include the parameter LANGUAGE="JavaScript" since LANGUAGE="VBScript" is also starting to be seen. HTML comment tags prevent the script contents from being printed out by browsers that don't handle JavaScript. The format is:

<SCRIPT LANGUAGE="JavaScript">

</SCRIPT>

While the LANGUAGE parameter in the SCRIPT tag is optional, it is a good idea to include it, since other SCRIPT types may be used in the future.

For a more complete description of JavaScript, you can visit Netscape's JavaScript Authoring Guide.

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C Function	JavaScript Function
Function format: function_name (arg_type argument ) { statement ; statement ; statement ; . . . return ; }	Function format same except: ; is optional, required only if more than one statement on a line arg_type is not used
function's arguments are typed	function's arguments are not typed
Called functions need prototype	No function prototypes
Pointers (addresses) can be used	No pointers allowed
*Variables must be explicitly typed as int, float, char, etc.*	Variables don't need to be typed, just assigned a value or declared with a var Variable Names A JavaScript identifier or name must start with a letter or underscore ("_"); subsequent characters can also include digits (0-9). Letters include the characters "A" through "Z" (uppercase) and the characters "a" through "z" (lowercase). JavaScript is case-sensitive (Abc is not the same variable as abc). Variable scope The scope of a variable is where you can reference it in a script. In JavaScript a variable can have either of two scopes: global: you can use the variable anywhere in the application. local: you can use the variable only within the current function. To declare a local variable inside a function, use the var keyword, for example: var total = 0 To declare a global variable, declare the variable by assignment, that is, simply assign the desired value to the variable (either in a function or outside a function). total = 0
No inherent objects	Many predefined objects with methods and properties
Must be compiled before execution	No compilation. Each statement is processed by interpreter every time it is used.