↑
Main Page

Logical AND

In this example, a Boolean variable (

found

) keeps track of the success of a search. When the item in

question is located,

found

is set to

true

, which causes

!found

to equal

false

, meaning that execution

will escape the

while

loop.

The logical NOT operator is also useful in determining the Boolean equivalent of an ECMAScript vari-

able. In order to do this, you use two logical NOT operators in a row. The first NOT returns a Boolean

value no matter what operand it is given. The second NOT negates that Boolean value and so gives the

true Boolean value of a variable.

var bFalse = false;

var sBlue = “blue”;

var iZero = 0;

var iThreeFourFive = 345;

var oObject = new Object;

document.write(“The Boolean value of bFalse is “ + (!!bFalse));

document.write(“<br />The Boolean value of sBlue is “ + (!!sBlue));

document.write(“<br />The Boolean value of iZero is “ + (!!iZero));

document.write(“<br />The Boolean value of iThreeFourFive is “ +

(!!iThreeFourFive));

document.write(“<br />The Boolean value of oObject is “ + (!!oObject));

Running this example yields the following output:

The Boolean value of bFalse is false

The Boolean value of sBlue is true

The Boolean value of iZero is false

The Boolean value of iThreeFourFive is true

The Boolean value of oObject is true

Logical AND

The logical AND operator in ECMAScript is indicated by the double ampersand (

var bTrue = true;

var bFalse = false;

var bResult = bTrue && bFalse;

Logical AND behaves as described in the following truth table:

Operand 1

Operand 2

Result

true

false

true

false

Logical AND can be used with any type of operands, not just Boolean values. When either operand is

not a primitive Boolean, logical AND does not always return a Boolean value:

Chapter 2

05_579088 ch02.qxd 3/28/05 11:35 AM Page 44