Logical AND (&&)

Logical AND (&&) evaluates operands from left to right, returning immediately with the value of the first falsy operand it encounters; if all values are truthy, the value of the last operand is returned.

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:

• false;
• null;
• NaN;
• 0;
• empty string ("" or '' or ``);
• undefined.

The AND operator preserves non-Boolean values and returns them as they are:

result = "" && "foo"; // result is assigned "" (empty string)
result = 2 && 0; // result is assigned 0
result = "foo" && 4; // result is assigned 4

Even though the && operator can be used with non-Boolean operands, it is still 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.

The logical AND expression is a short-circuit operator. As each operand is converted to a boolean, if the result of one conversion is found to be false, the AND operator stops and returns the original value of that falsy operand; it does not evaluate any of the remaining operands.

Consider the pseudocode below.

(some falsy expression) && expr

The expr part is never evaluated because the first operand (some falsy expression) is evaluated as falsy. If expr is a function, the function is never called. See the example below:

function A() {
  console.log("called A");
  return false;
}
function B() {
  console.log("called B");
  return true;
}

console.log(A() && B());
// Logs "called A" to the console due to the call for function A,
// && evaluates to false (function A returns false), then false is logged to the console;
// the AND operator short-circuits here and ignores function B

The AND operator has a higher precedence than the OR operator, meaning the && operator is executed before the || operator (see operator precedence).

true || false && false; // true
true && (false || false); // false
(2 === 3) || (4 < 0) && (1 === 1); // false

The following code shows examples of the && (logical AND) operator.

a1 = true && true; // t && t returns true
a2 = true && false; // t && f returns false
a3 = false && true; // f && t returns false
a4 = false && 3 === 4; // f && f returns false
a5 = "Cat" && "Dog"; // t && t returns "Dog"
a6 = false && "Cat"; // f && t returns false
a7 = "Cat" && false; // t && f returns false
a8 = "" && false; // f && f returns ""
a9 = false && ""; // f && f returns false

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)

As logical expressions are evaluated left to right, it is always possible to remove parentheses from a complex expression provided that certain rules are followed.

The following composite operation involving booleans:

bCondition1 || (bCondition2 && bCondition3)

is always equal to:

bCondition1 || bCondition2 && bCondition3

• Boolean
• Truthy
• Falsy