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Logical Operators

Logical operators are typically used with Boolean(logical) values; when they are, they return a Boolean value. However, the && and || operators actually return the value of one of the specified operands, so if these operators are used with non-Boolean values, they may return a non-Boolean value. The logical operators are described in the following table.

Operator Usage Description
&& expr1 && expr2 (Logical AND) Returns expr1 if it can be converted to false; otherwise, returns expr2. Thus, when used with Boolean values, && returns true if both operands are true; otherwise, returns false.
|| expr1 || expr2 (Logical OR) Returns expr1 if it can be converted to true; otherwise, returns expr2. Thus, when used with Boolean values, || returns true if either operand is true; if both are false, returns false.
! !expr1 (Logical NOT) Returns false if its single operand can be converted to true, otherwise returns true.

Examples of expressions that can be converted to false are those that evaluate to null, 0, the empty string(""), or undefined.

The following code shows examples of the && (logical AND) operator. 


a1=true && true         // t && t returns true
a2=true && false        // t && f returns false
a3=false && true        // f && t returns false
a4=false && (3 == 4)    // f && f returns false
a5="Cat" && "Dog"       // t && t returns Dog
a6=false && "Cat"       // f && t returns false
a7="Cat" && false       // t && f returns false

The following code shows examples of the || (logical OR)operator. 


o1=true || true         // t || t returns true
o2=false || true        // f || t returns true
o3=true || false        // t || f returns true
o4=false || (3 ==4)     // f || f returns false
o5="Cat" || "Dog"       // t || t returns Cat
o6=false || "Cat"       // f || t returns Cat
o7="Cat" || false       // t || f returns Cat

The following code shows examples of the ! (logical NOT)operator. 


n1=!true                // !t returns false
n2=!false               // !f returns true
n3=!"Cat"               // !t returns false

Short-circuit evaluation

As logical expressions are evaluated left to right, they are tested for possible "short-circuit" evaluation using the following rules:

  • false && anything is short-circuit evaluated false.
  • true || anything is short-circuit evaluated true.

The rules of logic guarantee that these evaluations are always correct. Note that the anything part of the above expressions is not evaluated, so any side effects of doing so do not take effect.

 

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