The NOT operator

The NOT operator (!) flips truthy and falsey values around. Truthy values becomes false while falsey values become true.

console.log(!22) // false
console.log(!false) // true

The NOT operator can be used to eliminate else statements, like this:

const str = ''

if (!str) {
  // Only do something if string is empty (NOT truthy)
}

Double negation

You may notice that developers sometimes use two NOT operators together:

!!someVariable

!! here is called a double negation. It does the following:

1. converts a truthy value to true
2. converts a falsey value to false

It works this way: if the value is truthy, convert it to false with the first ! NOT operator; then, convert false to true again with the second ! NOT operator (and vice versa).

Double negation is used to explicitly cast a truthy or falsey value into a boolean (true or false). You’ll almost never need it.

Exercise

What’s the result of each of these expressions?

1. !2550284
2. !true
3. !NaN
4. !{}
5. !!'Pandas are adorable!'
6. !!''