The logical AND (&&) (logical conjunction) operator for a set of boolean operands will be true if and only if all the operands are true. Otherwise it will be false.
More generally, the operator returns the value of the first operand encountered when evaluating from left to right, or the value of the last operand if they are all .
Syntax
x && y
Description
Logical AND (&&) evaluates operands from left to right, returning immediately with the value of the first
operand it encounters; if all values are
, the value of the last operand is returned.
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:
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.
Short-circuit evaluation
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
.
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
Operator precedence
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
Examples
Using AND
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
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 provided that certain rules are followed.
The following composite operation involving booleans:
bCondition1 || (bCondition2 && bCondition3)
is always equal to:
bCondition1 || bCondition2 && bCondition3