← Back tothis vs that

a vs !!a

Written byPhuoc Nguyen
25 Aug, 2023
In JavaScript, the '!' symbol is the logical NOT operator. It returns the opposite boolean value of the operand. If the operand is `true`, it returns `false`, and if the operand is `false`, it returns `true`.
However, when you use '!' twice, it becomes the logical NOT NOT operator, which is the same as using no operator at all. So, `!!a` simply returns the boolean value of `a`.
Here's an example to show the difference between using `a` and `!!a`:
const a = 0;
console.log(a); // 0
console.log(!a); // true
console.log(!!a); // false
In the example above, we set the value of `a` to 0, which is considered a falsy value in JavaScript.
In addition to 0, there are other values that are considered "falsy", including `false`, `NaN`, `undefined`, `null`, and `""` (an empty string).
When we apply the logical NOT operator on `a`, it returns `true` because 0 is a falsy value.
However, when we apply the logical NOT NOT operator on `a`, it returns `false` because the boolean value of 0 is `false`.
In general, using `a` is useful when you want to check if a value is truthy or falsy. On the other hand, using `!!a` is helpful when you want to get the boolean value of a variable.

See also

Questions? 🙋

Do you have any questions? Not just about this specific post, but about any topic in front-end development that you'd like to learn more about? If so, feel free to send me a message on Twitter or send me an email. You can find them at the bottom of this page.
I have a long list of upcoming posts, but your questions or ideas for the next one will be my top priority. Let's learn together! Sharing knowledge is the best way to grow 🥷.

Recent posts ⚡

Newsletter 🔔

If you're into front-end technologies and you want to see more of the content I'm creating, then you might want to consider subscribing to my newsletter.
By subscribing, you'll be the first to know about new articles, products, and exclusive promotions.
Don't worry, I won't spam you. And if you ever change your mind, you can unsubscribe at any time.
Phước Nguyễn