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Variables
You use variables as symbolic names for values in your application. The names of variables, called identifiers, conform to certain rules.
A JavaScript identifier must start with a letter, underscore (_), or dollar sign ($); 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).
Starting with JavaScript 1.5, you can use ISO 8859-1 or Unicode letters such as å and ü in identifiers. You can also use the \uXXXX Unicode escape sequences listed on the Unicode Escape Sequences page as characters in identifiers.
Some examples of legal names are Number_hits, temp99, and _name.
Declaring Variables
You can declare a variable in two ways:
- With the keyword var. For example,
var x = 42. This syntax can be used to declare both local and global variables. - By simply assigning it a value. For example,
x = 42. This always declares a global variable and generates a strict JavaScript warning. You shouldn't use this variant.
Evaluating Variables
A variable declared using the var statement with no initial value specified has the value undefined.
An attempt to access an undeclared variable will result in a ReferenceError exception being thrown:
var a;
print("The value of a is " + a); // prints "The value of a is undefined"
print("The value of b is " + b); // throws ReferenceError exception
You can use undefined to determine whether a variable has a value. In the following code, the variable input is not assigned a value, and the if statement evaluates to true.
var input;
if(input === undefined){
doThis();
} else {
doThat();
}
Not sure how the following is related to "Variables" section The undefined value behaves as false when used in a boolean context. For example, the following code executes the function myFunction because the myArray element is not defined:
var myArray = new Array(); if (!myArray[0]) myFunction();
When you evaluate a null variable, the null value behaves as 0 in numeric contexts and as false in boolean contexts. For example:
var n = null; print(n * 32); // prints 0
Variable Scope
When you declare a variable outside of any function, it is called a global variable, because it is available to any other code in the current document. When you declare a variable within a function, it is called a local variable, because it is available only within that function.
JavaScript does not have block statement scope; rather, it will be local to the code that the block resides within. For example the following code will print 0 instead of throwing if condition is false:
if (condition) {
var x = 5;
}
print(x ? x : 0);
Another unusual thing about variables in JavaScript is that you can refer to a variable declared later, without getting an exception:
print(x === undefined); // prints "true" var x = 3;
Global Variables
need links to pages discussing scope chains and the global object Global variables are in fact properties of the global object. In web pages the global object is window, so you can set and access global variables using the window.variable syntax.
Consequently, 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.
See Also
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