javascript

Chapter 3: Language Basics
In this example, the variable message is declared without initializing it. When compared with the literal
value of undefined , the two are equal. This example is identical to the following:
var message = undefined;
alert(message == undefined);
//true
Here the variable message is explicitly initialized to be undefined . This is unnecessary because, by
default, any uninitialized variable gets the value of undefined .
Generally speaking, you should never explicitly set a variable to be undefined.
The literal undefined value is provided mainly for comparison and wasn’t added
until ECMA-262 Third Edition to help formalize the difference between an empty
object pointer and an uninitialized variable.
Note that a variable containing the value of undefined is different from a variable that hasn ’ t been
defined at all. Consider the following:
var message;
//this variable is declared but has a value of undefined
//make sure this variable isn’t declared
//var age
alert(message); //”undefined”
alert(age); //causes an error
In this example, the first alert displays the variable message , which is undefined. In the second alert, an
undeclared variable called age is passed into the alert() function, which causes an error because the
variable hasn ’ t been declared. Only one operation can be performed on an undeclared variable: you can
call typeof on it.
The typeof operator returns “undefined” when called on an uninitialized variable, but it also returns
“undefined” when called on an undeclared variable, which can be a bit confusing. Consider this
example:
var message;
//this variable is declared but has a value of undefined
//make sure this variable isn’t declared
//var age
alert(typeof message); //”undefined”
alert(typeof age); //”undefined”
In both cases, calling typeof on the variable returns the string “undefined” . Logically, this makes
sense because no real operations can be performed with either variable even though they are technically
very different.
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