6. What are falsy values?

Falsy values are values considered in JavaScript falsy in context of conditions. There are 6 of them and they are undefined, null, NaN, 0, "" (empty string) and false.

It means converting it to Boolean will end with getting false.

let counter = 0;
if (0 || "" || undefined || null || false || NaN) {
  counter++;
}
console.info(counter); // 0
console.info([0, "", undefined, null, false, NaN].map(Boolean)); // [false, false, false, false, false, false]

Be careful - it doesn't mean all of these values are equal to false

console.info(0 == false); // true
console.info("" == false); // true
console.info(false == false); // true
console.info(NaN == false); // false
console.info(undefined == false); // false
console.info(null == false); // false