# Logical OR (||)

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 truthy. If a value can be converted to `false`, the value is so-called falsy.

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 `null` or `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)
``````

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