Learning JavaScript

The Logical Operators

Most of the examples so far in the book show a conditional expression that consists usually of one operator and two operands, such as the following:

if (sValue == 'test')

However, many times a conditional expression is dependent on several different conditions being met, each represented by an expression and combined through the use of one of JavaScript’s logical operators.

There are three logical operators—two binary and one unary. The first is the logical AND, represented by two ampersand characters, &&. When used in a conditional statement, the AND operator requires that expressions on both sides of the operator evaluate to true for the entire expression to evaluate to true:

var nValue = 10;
if ((nValue > 10) && (nValue <=100)) // true if nValue is greater than 10 and nValue is less than or equal to 100

The result of using this expression joined by the AND operator is false because the variable, nValue, is equal to 10, which means the first expression is false. If the first expression evaluates to false, the JavaScript engine won’t process the second expression because the entire statement is going to fail regardless.

The second operator is the logical OR operator, represented by two vertical lines, ||. When used in a conditional statement, the OR operator requires one or the other of its expressions on either side to be true in order for the entire expression to evaluate to true:

var nValue = 10; if ((nValue > 10) || (nValue <= 100)) // true if nValue is either ...