JavaScript Logical Operators
Although logical operators are usually used with Boolean (logical) values and return Boolean values, the && and || operators can be used with non-Boolean values, in which case they may return a non-Boolean value. <
Expressions that can be converted to false include those that evaluate to null, 0, the empty string (""), or undefined.
&& - Logical AND - returns its first operand if it can be converted to false. Otherwise, it returns the second operand.
|| - Logical OR - returns its first operand if it can be converted to true. Otherwise, it returns the second operand.
! - Logical NOT - returns false if its single operand can be converted to true. Otherwise, returns true. So, !true returns false, !false returns true, and !"Gog" returns false.
| a | b | Logical AND: a && b returns: | Logical OR: a || b returns: |
| true | true | true | true |
| true | false | false | true |
| false | true | false | true |
| false | (2==5) | false | false |
| "Gog" | "Magog" | Magog | Gog |
| false | "Magog" | false | Magog |
| "Gog" | false | false | Gog |
Short cuts:
false && anything always returns false,
true || anything always returns true.
When short cuts are used, the anything part is not evaluated, so there are no side effects from doing so.