c7ccd4c64b
Checking using !! to get the boolean value may be easier than setting up a conditional. Also linking to the truthy freeCodeCamp guide.
78 lines
2.3 KiB
Markdown
78 lines
2.3 KiB
Markdown
---
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title: Falsy Values
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---
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## Description
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A falsy value is something which evaluates to FALSE, for instance when checking a variable. There are only six falsy values in JavaScript: `undefined`, `null`, `NaN`, `0`, `""` or `''` (empty string), and the Boolean `false` of course. All other values are [truthy](https://github.com/freeCodeCamp/freeCodeCamp/edit/master/guide/english/javascript/truthy-values/index.md).
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## Checking for falsy values on variables
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It is possible to check for a falsy value in a variable with a simple conditional:
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```javascript
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if (!variable) {
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// When the variable has a falsy value the condition is true.
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}
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```
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You can also get the boolean value of a variable by using the bang operator (`!`) twice:
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```javascript
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!!variable // When the variable is falsy, a double bang (!!) will evaluate to the Boolean false.
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```
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## General Examples
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```javascript
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const string = ""; // <-- falsy
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const filledString = "some string in here"; // <-- truthy
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const zero = 0; // <-- falsy
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const numberGreaterThanZero; // <-- falsy
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const emptyArray = []; // <-- truthy, we'll explore more about this next
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const emptyObject = {}; // <-- truthy
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```
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## Fun With Arrays
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```javascript
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if ([] == false) // <-- truthy, will run code in if-block
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if ([]) // <-- truthy, will also run code in if-block
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if ([] == true) // <-- falsy, will NOT run code in if-block
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if (![]) // <-- falsy, will also NOT run code in if-block
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```
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## Caveat
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Be aware of the data type when evaluating a value in a Boolean context. If the data type of the value is meant to be a _number_, the truthy/falsy evaluation can result in an unexpected outcome:
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```javascript
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const match = { teamA: 0, teamB: 1 }
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if (match.teamA)
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// The following won't run due to the falsy evaluation
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console.log('Team A: ' + match.teamA);
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}
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```
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An alternative to the use case above is to evaluate the value using `typeof`:
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```javascript
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const match = { teamA: 0, teamB: 1 }
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if (typeof match.teamA === 'number')
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console.log('Team A: ' + match.teamA);
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}
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```
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## More Information
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- <a>**truthy**</a> | <a href='http://james.padolsey.com/javascript/truthy-falsey/' target='_blank' rel='nofollow'>Blog Post - Truthy & Falsey</a>
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- [ Falsy | Glossary | MDN](https://developer.mozilla.org/en-US/docs/Glossary/Falsy)
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- [Truthy and Falsy: When All is Not Equal in JavaScript](https://www.sitepoint.com/javascript-truthy-falsy/)
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