Logical Operators

JavaScript has the following logical operators:

• && and
• || or
• ! not

The logical operators work as expected when their operands are boolean expressions. For instance, x && y evaluates as truthy if both x and y evaluate to true. The operator also short-circuits; it does not evaluate y if x evaluates to false.

const isCitizen = true;
const age = 28;
console.log(isCitizen && age > 64); // false

Let’s revisit the earlier statement:

What if their operands are not boolean expressions? You might expect the operands to be converted to boolean values first, but JavaScript does something different.

The JavaScript && and || operators do not return a boolean. They always return one of their operands.

This is what actually happens:

console.log(true && "Ali"); // Ali
console.log(false && "Ali"); // false

console.log(true || "Ali"); // true
console.log(false || "Ali"); // Ali

JavaScript programmers rely on this behavior. You will find expressions such as

DEBUG && console.log("Some debugging message");

which is a short form of

if (DEBUG) {
  console.log("Some debugging message");
}

The variable DEBUG is presumably a boolean value, but it does not have to be. (See the next section!)