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The Undefined type

var sTemp = “test string”;

alert(typeof sTemp); //outputs “string”

alert(typeof 95); //outputs “number”

Calling

typeof

on a variable or value returns one of the following values:

“undefined”

if the variable is of the Undefined type.

“boolean”

if the variable is of the Boolean type.

“number”

if the variable is of the Number type.

“string”

if the variable is of the String type.

“object”

if the variable is of a reference type or of the Null type.

The Undefined type

As previously mentioned, the Undefined type has only one value,

undefined

. When a variable is

declared and not initialized, it is given the value of

undefined

by default.

var oTemp;

The previous line of code declares a variable named

oTemp

, which has no initialization value. This vari-

able is given a value of

undefined

, which is the literal representation of the Undefined type. You can

test that the variable is equal to the literal yourself by running this code snippet:

var oTemp;

alert(oTemp == undefined);

This code displays an alert with the word

“true”

, indicating that these two values are indeed equal.

You can also use the

typeof

operator to show that the variable has a value of

undefined

var oTemp;

alert(typeof oTemp); //outputs “undefined”

Note that a variable having the value of

undefined

is different from a value being undefined. However,

the

typeof

operator doesn’t actually distinguish between the two. Consider the following:

var oTemp;

//make sure this variable isn’t defined

//var oTemp2;

//try outputting

You may wonder why the

typeof

operator returns “object” for a value that is

null

This was actually an error in the original JavaScript implementation that was then

copied in ECMAScript. Today, it is rationalized that

null

is considered a place-

holder for an object, even though, technically, it is a primitive value.

ECMAScript Basics

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