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 |
---|---|---|
Logical AND (&& ) |
expr1 && expr2 |
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. |
Logical OR (|| ) |
expr1 || expr2 |
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. |
Logical NOT (! ) |
!expr |
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.
Even though the &&
and ||
operators can be used with operands that are not Boolean values, they can still be considered Boolean operators since their return values can always be converted to Boolean values.
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 to false.true || (anything)
is short-circuit evaluated to 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. Also note that the anything part of the above expression is any single logical expression (as indicated by the parentheses).
For example, the following two functions are equivalent.
function shortCircuitEvaluation() { doSomething() || doSomethingElse() } function equivalentEvaluation() { var flag = doSomething(); if (!flag) { doSomethingElse(); } }
However, the following expressions are not equivalent due to operator precedence, and stresses the importance of requiring the right hand operator to be a single expression (grouped if needed by parentheses).
false && true || true // returns true false && (true || true) // returns false
&&
)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
the following operation involving Booleans:
bCondition1 && bCondition2
is always equal to:
!(!bCondition1 || !bCondition2)
the following operation involving Booleans:
bCondition1 || bCondition2
is always equal to:
!(!bCondition1 && !bCondition2)
the following operation involving Booleans:
!!bCondition
is always equal to:
bCondition
As logical expressions are evaluated left to right, it is always possible to remove parentheses from a complex expression following some rules.
The following composite operation involving Booleans:
bCondition1 || (bCondition2 && bCondition3)
is always equal to:
bCondition1 || bCondition2 && bCondition3
The following composite operation involving Booleans:
bCondition1 && (bCondition2 || bCondition3)
is always equal to:
!(!bCondition1 || !bCondition2 && !bCondition3)
Created by Mozilla Contributors and licensed under CC-BY-SA 2.5