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Variables in JavaScript

By: aathishankaran in JavaScript Tutorials on 2007年03月21日 [フレーム]

A variable is the name given to a location in a computer's memory where data is stored.

The name of a JavaScript variable comprises one or more letters, digits, or underscores but cannot begin with a digit. Digits include 0 through 9. Letters include all uppercase characters, A through Z, and all lower case characters, a through z.

Declaring Variables

To let JavaScript know you are going to use an identifier as a variable, you must first declare it. To declare variables in JavaScript, use the keyword var followed by the new variable name. This action reserves the name as a variable to be used as a storage area for whatever data you might want to hold with it. In the examples that follow, notice that you can also declare more than one variable at a time by using a comma between variable names:

var internetAddress
var n
var i, j, k
var isMouseOverLink, helloMessage

Once a variable is declared, it is then ready to be filled with its first value. This initializing is done with the assignment operator, =. You can initialize a variable at the same time it is declared or at any point thereafter in your script. Assigning a value when the variable is declared can help you remember what type of value you originally meant the variable to hold.

Variable Types

When storing a piece of data (more commonly known as a value), JavaScript automatically categorizes it as one of the five JavaScript data types.

Scope of Variables

The scope of a variable refers to the area or areas within a program where a variable can be referenced. Suppose you embed one script in the head f an HTML document and another script (using another set of script tags) in the body of the same HTML document.

Local: A variable declared inside a function is local in scope. Only that function has access to the value that the variable holds. Each time the function is called, the variable is created. Likewise, each time the function ends, the variable is destroyed.

Another function can also declare a variable with the same name, but JavaScript considers it a different variable and does not address the same block of memory.

Global: If you want more than one function to share a variable, you declare the variable outside of any functions (but, of course, inside the <SCRIPT> tags). With this method, any part of your application, including all functions, can share one instance of a variable. I recommend that you declare global.

Constants

JavaScript does not supply any built-in constants. You could call true and false constants, but Netscape really categorizes them as keywords. A constant holds the same value throughout an application so that you can be sure it always carries the same value. Netscape might find a need for constants in the future, but for now, it works fine without them.

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