• Versions of JavaScript
• Embedding JavaScript
• JavaScript Grammar
• Variables and Data Types
• Operators
• Statements
• Functions
• Objects
• Event Handlers
• Conclusion
• Sample Script

While programming for any single version of JavaScript is relatively simple, writing code which functions across disparate versions, most notably Navigator 4 or above and MSIE 4 or above, is one of the major challenges and topics of discussion in JavaScript programming at this time.

<script language="JavaScript">
<!--
document.write("Hello World!");
//-->
</script>

<script language="JavaScript1.2">

<script language="JavaScript" src="corefunctions.js">
</script>

newVariable=5;

function newFunction()
{ var loop=1;
  total=0;
  ...additional statements... 
}

Type

A value, the data assigned to a variable, may consist of any sort of data. However, JavaScript considers data to fall into several possible types.
Depending on the type of data, certain operations may or may not be able to be performed on the values. For example, you cannot arithmetically multiply two string values. Variables can be these types:

Numbers	3 or 7.987, Integer and floating-point numbers. Integers can be positive, 0, or negative; Integers can be expressed in decimal (base 10), hexadecimal (base 16), and octal (base 8). A decimal integer literal consists of a sequence of digits without a leading 0 (zero). A leading 0 (zero) on an integer literal indicates it is in octal; a leading 0x (or 0X) indicates hexadecimal. Hexadecimal integers can include digits (0-9) and the letters a-f and A-F. Octal integers can include only the digits 0-7. A floating-point number can contain either a decimal point, an "e" (uppercase or lowercase), which is used to represent "ten to the power of" in scientific notation, or both. The exponent part is an "e" or "E" followed by an integer, which can be signed (preceded by "+" or "-"). A floating-point literal must have at least one digit and either a decimal point or "e" (or "E").
Booleans	True or False. The possible Boolean values are true and false. These are special values, and are not usable as 1 and 0. In a comparison, any expression that evaluates to 0 is taken to be false, and any statement that evaluates to a number other than 0 is taken to be true.
Strings	"Hello World !" Strings are delineated by single or double quotation marks. (Use single quotes to type strings that contain quotation marks.)
Objects	myObj = new Object();
Null	Not the same as zero - no value at all. A null value is one that has no value and means nothing.
Undefined	A value that is undefined is a value held by a variable after it has been created, but before a value has been assigned to it.

That said, JavaScript is a loosely typed language -- you do not have to specify the data type of a variable when you declare it, and data types are converted automatically as needed during script execution. By and large, you may simply assign any type of data to any variable. The only time data typing matters is when you need to perform operations on the data. Certain operators behave differently depending on the type of data being deal with. For example, consider the + operator:

arithmetic or computational

Arithmetic operators take numerical values (either literals or variables) as their operands and return a single numerical value. The standard arithmetic operators are:

+	Addition
-	Subtraction
*	Multiplication
/	Division
%	Modulus: the remainder after division; e.g. 10 % 3 yields 1.
++	Unary increment: this operator only takes one operand. The operand's value is increased by 1. The value returned depends on whether the ++ operator is placed before or after the operand; e.g. ++x will return the value of x following the increment whereas x++ will return the value of x prior to the increment.
--	Unary decrement: this operator only takes one operand. The operand's value is decreased by 1. The value returned depends on whether the -- operator is placed before or after the operand; e.g. --x will return the value of x following the decrement whereas x-- will return the value of x prior to the decrement.
-	Unary negation: returns the negation of operand.

Comparison

A comparison operator compares its operands and returns a logical value based on whether the comparison is true or not. The operands can be numerical or string values. When used on string values, the comparisons are based on the standard lexicographical (alphabetic) ordering.

==	"Equal to" returns true if operands are equal.
!=	"Not equal to" returns true if operands are not equal.
>	"Greater than" returns true if left operand is greater than right operand.
>=	"Greater than or equal to" returns true if left operand is greater than or equal to right operand.
<	"Less than" returns true if left operand is less than right operand.
<=	"Less than or equal to" returns true if left operand is less than or equal to right operand.

Boolean

Boolean operators are typically used to combine multiple comparisons into a conditional expression. For example, you might want to test whether (total>100) AND (stateTax=true). A boolean operator takes two operands, each of which is a true or false value, and returns a true or false result.

&&	"And" returns true if both operands are true.
||	"Or" returns true if either operand is true.
!	"Not" returns true if the negation of the operand is true (e.g. the operand is false).

String

Strings can be compared using the comparison operators. Additionally, you can concatenate strings using the + operator.

Assignment

The assignment operator (=) lets you assign a value to a variable. You can assign any value to a variable, including another variable (whose value will be assigned). Several shorthand assignment operators allow you to perform an operation and assign its result to a variable in one step.

=	Assigns the value of the righthand operand to the variable on the left. Example: total=100; Example: total=(price+tax+shipping)
+= (also -=, *=, /=)	Adds the value of the righthand operand to the lefthand variable and stores the result in the lefthand variable. Example: total+=shipping (adds value of shipping to total and assigned result to total)
&= (also |=)	Assigns result of (lefthand operand && righthand operand) to lefthand operand.

Special

Several JavaScript operators, rarely used, fall into no particular category. These operators are summarized below.

Conditional operator (condition) ? trueVal : falseVal	Assigns a specified value to a variable if a condition is true, otherwise assigns an alternate value if condition is false. Example: preferredPet = (cats > dogs) ? "felines" : "canines" If (cats>dogs), preferredPet will be assigned the string value "felines," otherwise it will be assigned "canines".
typeof operand	Returns the data type of operand. Example -- test a variable to determine if it contains a number: if (typeof total=="number") ...

Regular expressions (Netscape & MSIE 4)

New to JavaScript 1.2 is support for regular expressions, which are defined patterns used to match character combinations appearing in string values. Regular expressions are very powerful, potentially allowing you to search for any conceivable character pattern. However, they can also be quite complex to construct. Because regular expressions are widely supported in all high-level development environments, it is advised that you consider learning about regular expressions as a subject unto itself.

Two detailed explanations of regular expressions can be found at

• Netscape's JavaScript Guide: Chapter 9 Expressions and Operators
• PERL5 Regular Expression Description