Logical Operators in JavaScript
JavaScript has 3 logical operators:
!: Logical NOT&&: Logical AND||: Logical OR
Logical NOT
The logical not operator ! checks whether a value is truthy or falsy. In other words, !v === false unless v is one
of the below 7 values:
// Falsy values means `!v === true`
!false; // true
!0; // true
!''; // true
!undefined; // true
!null; // true
!Number('abc'); // true
// All other values are "truthy", meaning `!v === false`
!1; // false
!({}); // false
!({ valueOf: () => 0 }); // false
!' '; // falseThe not operator always returns a boolean, unlike || and &&. A common
way to convert a value v to a boolean is to use the logical not operator twice:
// Convert `v` to a boolean
!!v;Logical OR
The logical or operator || operates on 2 values, and returns a truthy
value if at least one of the values is truthy.
// True if `a` or `b` is truthy. Will only be `false` if
// both `a` and `b` are falsy.
!!(a || b);The logical or operator is most often used in if statements to execute
the if block if one of multiple conditions is true. But, the logical
or operator does not necessarily return a boolean:
- If
ais truthy,(a || b) === a - If
ais falsy,(a || b) === b
This works neatly with multiple or operators: a || b || c || d returns
the leftmost value that is truthy. Because of this, logical or is often
used for default values:
const defaultNumReps = 2;
function repeat(str, numReps) {
// If `numReps` is null, undefined, or 0, this sets
// `numReps` to the default value `2`
numReps = numReps || defaultNumReps;
let ret = '';
for (let i = 0; i < numReps; ++i) {
ret += str;
}
return ret;
}Logical AND
The logical and operator && operates on 2 values, and returns falsy
if at least one of the values is falsy:
// Only true if both `a` and `b` are truthy.
!!(a && b);The logical and operator is most often used in if statements to execute
the if block if multiple conditions are all true. But, like the logical
or operator, the logical and operator doesn't necessarily return a boolean:
- If
ais falsy,(a && b) === a - If
ais truthy,(a && b) === b
Given a chain of multiple logical and operators a && b && c && d, JavaScript
returns the left-most falsy value.