JavaScript Reference Syntax

JavaScript Syntax

HTML Tags

JavaScript code is published within the HTML of the Web page, delimited by the script tags:
<script language="javascript"> ... </script>

Coding

Everything written between the script tags is the JavaScript code itself. Lines of the code are terminated with a semi-colon (optional on the last line within the script):
    <script language="javascript">
        sText="<b>Generated by </b><i>JavaScript</i>.";
        document.write(sText);
    </script>
Generates:

Comments

Comments can be incorporated into scripts using the following methods:

`// text of comment`	everything following the double slashes is ignored
`/* text of comment */`	everything between the '/' and '/' is ignored

Variable Declaration

The working data within a script is made up of variables which are either in-built types of data, such as numeric or string values, or they are instances of an object type that is defined within the language. In either case, the naming of the variable is important as it must not clash with any other element that is known to the script, another variable or any of the in-built names (objects, methods, etc.- the reserved words). Variable names cannot contain spaces, but they can use under-scores.

Declaration of a non-object variable is done as follows:
var variable = value;

A variable can be numeric or text, depending on the type of data that is assigned to it. The numeric value of a text variable is zero.

The context within which a variable is declared is all important. Defining a variable within the <head> tags but outside of a function ensures that it is visible to all JavaScript code within the document.

Assignment

Values are assigned to variables (including object instances) using the equals sign:
variable = value;

Object Instantiation

The creation of objects is acheived as follows, where object is the type of object to be created and instance is the name of this instance of the object (the name that will be used to reference this instance in future):
instance = new object(...);
For example, to create a Date object that corresponds to the current date and time:
today = new Date();

Method Invocation

Actions are performed on objects by suffixing the object instance with a dot ('.') and the method name and enclosing any parameters in brackets. Any value returned by the method can be assigned to a variable as above:
variable = instance.method(parameters);

Object Properties

The properties of an object correspond to values of variables that are held inside of the object itself. Each instance of the same type of object will exhibit the same range of properties, but the values of each may be different. The current value of each property is accessed in a similar way to methods above:
variable = object.property; // Retrieve the value
object.property = variable; // Update the value

Function Declaration

Functions allow self-contained blocks of code to be written once and used throughout the document as and where necessary. This code must be placed within the <head> section of the document so that it is available throughout the body. The comment structure in the following template example demonstrates how to ensure that the code is not visible to browsers that do not support JavaScript:

<SCRIPT LANGAUGE="javascript">
        
    </SCRIPT>

Conditional structure

JavaScript supports the following structure that allows conditional control over the sequence in which the code is executed:
    if (condition)
    { statements }
    else
    { statements }
If the 'statements' consist of a single code phrase, the delimiting curly brackets can be left out. If no curly brackets open immediately after the 'if' or 'else' statements, only the immediately following code phrase is treated as part of the conditional structure.

The condition may be any combination of the following:
    variable == value   // or other conditional-operation
    object.property     // where the value is equivalent to a boolean
    object.method()     // return value is boolean

Loops

There are two distinct types of loops that are available, both of which are identical to their Java equivalent.

'For' loops are driven by a counter that has an the following structure:
for (initial value; condition; step operation)
{ statements; }

For instance, the following:
for (i=0; i < 3; i++)
document.write(i+"<br>"); Produces:

'While' loops perform a very similar role, except that the loop is only driven by the condition that determines whether another iteration is required and the initialisation and step operation can be isolated:
while (condition)
{ statements; }

Equivalent to the above:
var i = 0, max = 2;
while (i <= max)
{ document.write(i+"<br>"); i++; } Produces:

Arrays

The declaration and initialisation of an array takes the following form:
    variable=new Array();
    variable[0]="0";
    variable[1]="1";

There are a number of pre-defined arrays in the object hierarchy within JavaScript:
document.form_name is equivalent to document.forms[n]
document.image_name is equivalent to document.images[n]