# Logical OR (||)

The logical OR (`||`) operator (logical disjunction) 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

``````expr1 || expr2
``````

## Description

If `expr1` can be converted to `true`, returns `expr1`; else, returns `expr2`.

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 operator precedence cannot apply``````

## Examples

### Using OR

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"
o8 = ''    || false      // f || f returns false
o9 = false || ''         // f || f returns ""
o10 = 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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