Professional JavaScript For Web Developers

ECMAScript Basics
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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, null is returned.
If either operand is NaN, 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 undefined.
The value of variable bTrue is true, so the logical AND operator continued on to evaluate variable
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 a 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 true true
true false true
false true true
false false false
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