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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.

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