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Logical OR

If one operand is an object and one is a Boolean, the object is returned.

If both operands are objects, the second operand is returned.

If either operand is

null

is returned.

If either operand is

NaN

is returned.

If either operand is

undefined

, an error occurs.

Just as in Java, logical AND is a short-circuited operation, meaning that if the first operand determines

the result, the second operand is never evaluated. In the case of logical AND, if the first operand is false,

no matter what the value of the second operand, the result can’t be equal to true. Consider the following

example:

var bTrue = true;

var bResult = (bTrue && bUnknown); //error occurs here

alert(bResult); //this line never executes

This code causes an error when the logical AND is evaluated because the variable

bUnknown

is unde-

fined. The value of variable

bTrue

true

, so the logical AND operator continued on to evaluate vari-

able

bUnknown

. When it did, an error occurred because

bUnknown

is undefined and, therefore, cannot

be used in a logical AND operation. If this example is changed so that

is set to

false

, the error won’t

occur:

var bFalse = false;

var bResult = (bFalse && bUnknown);

alert(bResult); //outputs “false”

In this code, the script writes out the string

“false”

, the value returned by the logical AND operator.

Even though the variable

bUnknown

is undefined, it never gets evaluated because the first operand is

false

. You must always keep in mind short-circuiting when using logical AND.

Logical OR

The logical OR operator in ECMAScript is the same as in Java, using the double pipe (

var bTrue = true;

var bFalse = false;

var bResult = bTrue || bFalse;

Logical OR behaves as described in the following truth table:

Operand 1

Operand 2

Result

true

false

true

false

true

false

ECMAScript Basics

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