JavaScript Variables
Variables are used as symbolic names for values in your application. The name (JavaScript Identifier) you give then must conform to the following rules:
Names must begin with:
- a letter (A-Z or a-z) or an underscore ("_")
Subsequent characters may be:
- digits (0-9), letters (A-Z and/or a-z) or underscores ("_")
My_variable, year21, _variabubble.
Declaring Variables
To declare a variable, either:
- use the keyword var, e.g.
var age = 21, or - assign it a value, e.g.
age = 21.
Unassigned Variables
If a variable or array element has not been assigned a value, it takes the value undefined. If it was declared without var, evaluating the variable results in a runtime error. If it was declared with var, evaluating the variable results in the undefined value, or NaN in numeric contexts. So, for example:
function f1() { return y - 2; } f1() //Causes runtime errorIn practice, got invalid character in IE, no error in Netscape 4.6function f2() { return var y - 2; } f2() //returns NaNIn practice, got invalid character in IE, no error in Netscape 4.6
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){ // In NN4.6 input is undefined as
document.writeln("Undefined"); // I haven't assigned a value.
} else { // In IE4 you get an error as
document.writeln("Defined"); // 'undefined' is undefined.
}When used as a Boolean value, the undefined value behaves as false, for example:
anotherArray=new Array()
if (!anotherArray[0])
document.writeln("Undefined")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
n * 32 //returns 0