JavaScript » Constants » undefined
Refers to a variable that has never been declared or has not been assigned a value.
In ECMAScript, Undefined is classified as a primitive value. Your ability
to use Undefined will be extremely dependent upon the type and version of
your browser.
There are two definitions for Undefined. It can
refer to a variable that has never been declared. Or it can refer to a variable
that has been declared, but has not been assigned a value. The ECMA-262 standard
uses the second version to define Undefined.
Undefined is
also a type. You can use the typeof operator to determine the
type of a variable and it will return a type of "undefined" for an
Undefined variable.
For this example, the value NotThere has not
been declared. (Note that it is optional to enclose the argument for the
typeof operator inside a pair of parenthesis.)
NOTE:
In Internet Explorer, if you attempt to utilize an undefined variable, you will get a runtime error message.
Examples
Code:
document.write("NotThere is of type = " + typeof NotThere) Output:
NotThere is of type = undefinedExplanation:
For this example, the value NotThere has not been declared. (Note that it is optional to enclose the argument for the typeof operator inside a pair of parenthesis.)
Code:
var IsThere;
document.write("IsThere is of type = " + typeof IsThere)
Output:
IsThere is of type = undefined Explanation:
In this example, the value IsThere has been declared, but it is not assigned a value.
