38 lines
1.2 KiB
Markdown
38 lines
1.2 KiB
Markdown
---
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title: What does the double negation operator do in JavaScript?
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shortTitle: Double negation operator
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type: question
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language: javascript
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tags: [type]
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author: chalarangelo
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cover: memories-of-pineapple-2
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excerpt: You've probably come across the double negation operator (`!!`) before, but do you know what it does?
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dateModified: 2022-07-26T05:00:00-04:00
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---
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JavaScript's negation operator (`!`) is a unary operator, used to invert the truth value of its operand. When used twice, known as the double negation operator (`!!`), it can be used to convert a value to a boolean.
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```js
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const x = 1;
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const y = null;
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!!x; // true
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!!y; // false
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```
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Using the double negation operator is functionally equivalent to using the `Boolean()` function, which we explored in depth in a [previous article](/js/s/boolean-function). In terms of readability and usability, I would still suggest using the `Boolean()` function. It conveys the intent of the operation more clearly, and it's easier to understand at a glance.
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```js
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const x = 1;
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const y = null;
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Boolean(x); // true
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Boolean(y); // false
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const values = [0, 0, 2, 0, 3];
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// Kinda readable, but not great
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values.filter(x => !!x); // [2, 3]
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// Arguably more readable
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values.filter(Boolean); // [2, 3]
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```
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