Variables

You use variables as symbolic names for values in your application. You give variables names by which you refer to them and which must conform to certain rules.

A JavaScript identifier, or name, must start with a letter or underscore ("_"); subsequent characters can also be digits (0-9). Because JavaScript is case sensitive, letters include the characters "A" through "Z" (uppercase) and the characters "a" through "z" (lowercase).

Some examples of legal names are Number_hits, temp99, and _name.

You can declare a variable in two ways:

• By simply assigning it a value; for example, x = 42
• With the keyword var; for example, var x = 42

When you set a variable identifier by assignment outside of a function, it is called a global variable, because it is available everywhere in the current document. When you declare a variable within a function, it is called a local variable, because it is available only within the function. Using var is optional, but you need to use it if you want to declare a local variable inside a function that has already been declared as a global variable.

You can access global variables declared in one window or frame from another window or frame by specifying the window or frame name. For example, if a variable called phoneNumber is declared in a FRAMESET document, you can refer to this variable from a child frame as parent.phoneNumber.

Data type conversion

JavaScript is a loosely typed language. That means 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. So, for example, you could define a variable as follows:

var answer = 42

And later, you could assign the same variable a string value, for example,

answer = "Thanks for all the fish..."

Because JavaScript is loosely typed, this assignment does not cause an error message.

In expressions involving numeric and string values, JavaScript converts the numeric values to strings. For example, consider the following statements:

x = "The answer is " + 42
y = 42 + " is the answer."

The first statement returns the string "The answer is 42." The second statement returns the string "42 is the answer."

Javascript Review

JavaScript is a compact, object-based scripting language for developing client and server Internet applications. Netscape Navigator interprets JavaScript statements embedded in an HTML page, and LiveWire enables you to create server-based applications similar to Common Gateway Interface (CGI) programs.
JavaScript is Netscape's cross-platform, object-based scripting language for client and server applications. There are two types of JavaScript:

• Navigator JavaScript, also called client-side JavaScript
• LiveWire JavaScript, also called server-side JavaScript

JavaScript is a language. Client and server JavaScript differ in numerous ways, but they have the following elements in common:

• Keywords, statement syntax, and grammar
• Rules for expressions, variables, and literals
• Underlying object model (although Navigator and LiveWire have different object frameworks)
• Built-in objects and functions