The logical OR (||) (logical disjunction) operator for a set of operands
is true if and only if one or more of its operands is true. It is typically used with
boolean (logical) values. When it is, it returns a Boolean value. However,
the || operator actually returns the value of one of the specified
operands, so if this operator is used with non-Boolean values, it will return a
non-Boolean value.
Syntax
x || y
Description
If x can be converted to true, returns
x; else, returns y.
If a value can be converted to true, the value is so-called
. If a value can be converted to false, the value is
so-called
.
Examples of expressions that can be converted to false are:
null;NaN;0;- empty string (
""or''or``); undefined.
Even though the || operator can be used with operands that are not Boolean
values, it can still be considered a boolean operator since its return value can always
be converted to a boolean primitive.
To explicitly convert its return value (or any expression in general) to the
corresponding boolean value, use a double NOT operator or the Boolean()
constructor.
Short-circuit evaluation
The logical OR expression is evaluated left to right, it is tested for possible "short-circuit" evaluation using the following rule:
(some truthy expression) || expr is short-circuit evaluated to
the truthy expression.
Short circuit means that the expr part above is not
evaluated, hence any side effects of doing so do not take effect (e.g., if
expr is a function call, the calling never takes place). This
happens because the value of the operator is already determined after the evaluation of
the first operand. See example:
function A() {
console.log("called A");
return false;
}
function B() {
console.log("called B");
return true;
}
console.log(B() || A());
// Logs "called B" due to the function call,
// then logs true (which is the resulting value of the operator)
Operator precedence
The following expressions might seem equivalent, but they are not, because the
&& operator is executed before the || operator
(see operator precedence).
true || false && false; // returns true, because && is executed first
(true || false) && false; // returns false, because grouping has the highest precedence
Examples
Using OR
The following code shows examples of the || (logical OR) operator.
true || true; // t || t returns true
false || true; // f || t returns true
true || false; // t || f returns true
false || 3 === 4; // f || f returns false
"Cat" || "Dog"; // t || t returns "Cat"
false || "Cat"; // f || t returns "Cat"
"Cat" || false; // t || f returns "Cat"
"" || false; // f || f returns false
false || ""; // f || f returns ""
false || varObject; // f || object returns varObject
Note: If you use this operator to provide a default value to some variable, be aware that any falsy value will not be used. If you only need to filter out
nullor undefined, consider using the nullish coalescing operator.
Conversion rules for booleans
Converting AND to OR
The following operation involving booleans:
bCondition1 && bCondition2
is always equal to:
!(!bCondition1 || !bCondition2)
Converting OR to AND
The following operation involving booleans:
bCondition1 || bCondition2
is always equal to:
!(!bCondition1 && !bCondition2)
Removing nested parentheses
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)