Ifx can be converted totrue, returnsx; else, returnsy.

If a value can be converted totrue, the value is so-calledtruthy. If a value can be converted tofalse, the value isso-calledfalsy.

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 Booleanvalues, it can still be considered a boolean operator since its return value can alwaysbe converted to aboolean primitive.To explicitly convert its return value (or any expression in general) to thecorresponding boolean value, use a doubleNOT operator or theBoolean()constructor.

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 tothe truthy expression.

Short circuit means that theexpr part above isnotevaluated, hence any side effects of doing so do not take effect (e.g., ifexpr is a function call, the calling never takes place). Thishappens because the value of the operator is already determined after the evaluation ofthe 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)

The following expressions might seem equivalent, but they are not, because the&& operator is executed before the|| operator(seeoperator precedence).

true || false && false; // returns true, because && is executed first(true || false) && false; // returns false, because grouping has the highest precedence

The following code shows examples of the|| (logical OR) operator.

true || true; // t || t returns truefalse || true; // f || t returns truetrue || false; // t || f returns truefalse || 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 falsefalse || ""; // f || f returns ""false || varObject; // f || object returns varObject

Converting AND to OR

The following operation involvingbooleans:

bCondition1 && bCondition2

is always equal to:

!(!bCondition1 || !bCondition2)

Converting OR to AND

The following operation involvingbooleans:

bCondition1 || bCondition2

is always equal to:

!(!bCondition1 && !bCondition2)

As logical expressions are evaluated left to right, it is always possible to removeparentheses from a complex expression following some rules.

The following composite operation involvingbooleans:

bCondition1 && (bCondition2 || bCondition3)

is always equal to:

!(!bCondition1 || !bCondition2 && !bCondition3)

• Nullish coalescing operator (??)
• Boolean
• Truthy
• Falsy