Booleans

A boolean has only two possible values: true and false. Booleans are perfect for representing yes/no or on/off states, and they are commonly the result of a comparison.

const isLoggedIn = true;
const hasPermission = false;
console.log(isLoggedIn, hasPermission);

Comparison operators like ===, !==, >, <, >=, and <= evaluate to a boolean, making them the most common way booleans show up in real code.

console.log(5 > 3);    // true
console.log(5 === 10); // false

JavaScript also treats every value as either "truthy" or "falsy" whenever a boolean is expected, such as inside an if condition. There are exactly six falsy values: false, 0, "" (empty string), null, undefined, and NaN. Every other value — including "0" as a string, or an empty array [] — is truthy.

if ("") {
  console.log("This won't run");
}
if ("hello") {
  console.log("This will run"); // a non-empty string is truthy
}

A value that behaves like false in a boolean context, such as 0 or an empty string, is called ____.

Which of these values is falsy in JavaScript?

An empty string '' is falsy, but a string containing any character, like 'no', is truthy.