Why would I use !! in JavaScript?

Question

While reading lodash source code, I saw:

this.__chain__ = !!chainAll;

Why would one use !! on the chainAll parameter?

I assume this is a safer way to detect falsy values or dealing w/ different JavaScript versions, but would like to know the scenario it protects.

It converts a value to its boolean counterpart. Now you *know* that it's a boolean instead of it being a string or something else. — gen_Eric, Mar 31 '14 at 15:45
Easy to search for btw: [`[javascript] "!!"`](http://stackoverflow.com/search?q=%5Bjavascript%5D+%22%21%21%22). — Felix Kling, Mar 31 '14 at 15:47

nanobash · Answer 1 · 2014-03-31T15:51:11.893

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The !! construct is a simple way of turning any JavaScript expression into its Boolean equivalent. For exmaple: !!"something" === true, while !!0 === false

DEMO

edited Mar 31 '14 at 15:51

answered Mar 31 '14 at 15:45

nanobash

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score 0 · Answer 2 · answered Mar 31 '14 at 15:48

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JS use dynamic type for variable. Thanks to this trick you convert the variable into a boolean if it was not a boolean, an it doesn't modify the value if it was a boolean

answered Mar 31 '14 at 15:48

Gwenc37

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Why would I use !! in JavaScript?

2 Answers2