Logical AND : Boolean Operators « Operators « JavaScript Tutorial

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The logical AND operator in JavaScript is indicated by the double ampersand (&&):

var bTrue = true;
var bFalse = false;
var bResult = bTrue && bFalse;

Logical AND behaves as described in the following truth table:

Logical AND can be used with any type of operands, not just Boolean values. When either operand is not a primitive Boolean, logical AND does not always return a Boolean value:

If one operand is an object and one is a Boolean, the object is returned.

If both operands are objects, the second operand is returned.

If either operand is null, null is returned.

If either operand is NaN, NaN is returned.

If either operand is undefined, an error occurs.