Javascript ! and !! differences

Question

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What is the !! operator in JavaScript?

What is the difference between these two operators? Does !! have special meaning, or does it simply mean you are doing two '!' operations. I know there are "Truth" and "Truthy" concepts in Javascript, but I'm not sure if !! is meant for "Truth"

score 8 · Answer 1 · answered May 30 '11 at 14:12

8

!! is just double !

!true // -> false
!!true // -> true

!! is a common way to cast something to boolean value

!!{}  // -> true
!!null // -> false

answered May 30 '11 at 14:12

bjornd

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Andrew Whitaker · Accepted Answer · 2011-05-30T14:19:31.287

7

Writing !! is a common way of converting a "truthy" or "falsey" variable into a genuine boolean value.

For example:

var foo = null;

if (!!foo === true) {
    // Code if foo was "truthy"
}

After the first ! is applied to foo, the value returned is true. Notting that value again makes it false, meaning the code inside the if block is not entered.

edited May 30 '11 at 14:19

answered May 30 '11 at 14:14

Andrew Whitaker

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@For clarity, perhaps "into a *genuine* boolean value". `!!0` is, of course, a *false* boolean value... – lonesomeday May 30 '11 at 14:15
So why not just do `if(foo) {}`? Wouldn't this also do the same? – void.pointer May 30 '11 at 14:18
@Robert: Yes it would. I should probably make it `if(!!foo === true)` – Andrew Whitaker May 30 '11 at 14:19
@Robert: In this case you don't need the `!!`. But it is handy whenever you need a real boolean representation of some value. – Felix Kling May 30 '11 at 14:20

Javascript ! and !! differences

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