a vs !!a
Written byPhuoc Nguyen
Created
25 Aug, 2023
Category
JavaScript
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`:js
const a = 0;
console.log(a); // 0
console.log(!a); // true
console.log(!!a); // falseIn 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
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Phước Nguyễn