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The Null type

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

alert(typeof oTemp2); //outputs “undefined”

The previous code outputs

“undefined”

for both variables, even though only one of them (

oTemp2

) is

undefined. If you try to use

oTemp2

with any operator other than

typeof

, it causes an error because

operators can only be applied to defined variables. For example, this causes an error:

//make sure this variable isn’t defined

//var oTemp2;

//try outputting

alert(oTemp2 == undefined); //causes error

The value

undefined

is also returned when a function doesn’t explicitly return a value, as in the

following:

function testFunc() {

//leave the function blank

}

alert(testFunc() == undefined); //outputs “true”

The Null type

Another type with just one value, the Null type, has only the special value

null

, which is also its literal.

The value

undefined

is actually a derivative of the value

null

, so ECMAScript defines them as equal to

each other.

alert(null == undefined); //outputs “true”

Even though the values are both true, they are considered to have different meanings. Whereas

undefined

is the value assigned when a variable is declared and not initialized,

null

is the value used

to represent an object that doesn’t exist (which I touched upon briefly in the discussion of the

typeof

operator). If a function or method is supposed to return an object, it usually returns

null

when the

object isn’t found.

The Boolean type

The Boolean type is one of the most frequently used in the language. It has two values,

true

and

false

(which are also the two Boolean literals). Even though

false

isn’t equal to

is converted to

false

when necessary, making it safe to use either in a Boolean statement.

var bFound = true;

var bLost = false;

The Number type

The most unique type defined in ECMA-262 is the Number type. The Number type can represent both

32-bit integer and 64-bit floating-point values. A Number type literal is considered any number entered

Chapter 2

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