Manual Double Bang!

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What you can do is convert each to a truth value and compare them: Tons of great answers here, but if you've read down this far, this helped me to 'get it'. Open the console on Chrome etc , and start typing:. Naturally, these are all the same as merely typing!! The first bang forces the js engine to run Boolean x but also has the side effect of inverting the value. So the second bang undoes the side effect. That's why we use another! So it's like using Boolean function in javascript, but easy and shorter way to convert a value to boolean:.

Some operators in JavaScript perform implicit type conversions, and are sometimes used for type conversion. In boolean 0 means false and 1 means true. In boolean empty strings, null and undefined values mean false and if there is a string it means true. If a is defined, a is true ,! Use logical not operator two times it means! After seeing all these great answers, I would like to add another reason for using!! Currenty I'm working in Angular TypeScript and I want to return a boolean as false when my user is not authenticated.

If he isn't authenticated, the token-string would be null or "". I can do this by using the next block of code:. If vertical is strictly not undefined, return the boolean value of vertical and set it to this. If not, don't set a value for this. Falsey values in javascript coerce to false , and truthy values coerce to true. Falsey and truthy values can also be used in if statements and will essentially "map" to their corresponding boolean value. However, you will probably not find yourself having to use proper boolean values often, as they mostly differ in output return values. Although this may seem similar to casting, realistically this is likely a mere coincidence and is not 'built' or purposely made for and like a boolean cast.

So let's not call it that. To be concise, it looks something like this: Whereas, null is falsey , so! Overall, the context that you saw simply adjusts this. In other words, if vertical is defined; this.

Javascript “Bang, Bang. I shot you down” - Use of double bangs (!!) in Javascript.

I guess that in itself is an example of how you would use!! Run this example and fiddle around with the vertical value in the input. See what the result coerces to so that you can fully understand your context's code. In the input, enter any valid javascript value. Remember to include the quotations if you are testing out a string. However, you might want to take a look at the non-DOM-related javascript code though the use of the Example constructor and the vertical variable.

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The context in which I saw this was, this. Just for the record, don't do what is quoted there. It converts a nonboolean to an inverted boolean for instance,! An easy way to describe it is: This is used to convert truthy values to boolean true, and falsy values too boolean false. Micah Snyder be careful that in JavaScript it's better to use boolean primitives instead of creating objects that box booleans with new Boolean. Here's an example to see the difference: It's a horribly obscure way to do a type conversion.

So you're converting a value to a boolean, then inverting it, then inverting it again.

Practical uses

Lightness Races in Orbit k 48 Is the "much easier to understand" variant really much easier to understand here? The check against 0 is not an actual check against 0, but a check against the somewhat weird list of values Javascript considers equal to 0. My brain doesn't have any problem decoding!! Consider these examples, especially the 3rd example and onward: Salman A k 62 Yes I know, but consider the fact that most native constructors String , Number , Date , etc are meant to be callable as functions too, yet in this case the result is different!

SalmanA I also hope javascript makes sense sometimes: Here are the three approaches: But wait, there's more! There are two ways to do this: Benny 2, 1 12 TomAuger if statements do cast boolean against falsey values, but say you want to actually set a boolean flag on an object - it won't cast it like an if statement does.

Another example is maybe all admins are id of 0 and non-admins are id 1 or higher. To get true if someone is not an admin you could do person. Few use cases, but it's concise when there is. Or in the case of a boolean value on the right, it does nothing. White Sep 10 '09 at In the case of a boolean the right-most! Net effect is that there is no change, but most engines will generate op codes for the double negation. But what is the point?


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Javascript already knows this is false. Why is it better to say if!! This is an easy way to test existence. Christoph k 30 But then you can run into issues with instanceof.

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This is useful for a little hack when you need to evaluate strings with "0" as false instead of true. DiegoDD why would you choose!! Crockford is moving his site around a bit; that link is liable to die at some point for a little on why: WScript, for instance, wins the prize. In this case, both "have a truthiness of falsy".

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EDIT 24 Oct But if you see strange 0 s in your rendered JSX, think loose falsy management. So would you say the!!

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Nope, you wouldn't need it. Truthiness and true "externally" operate exactly the same in an if. I keep trying, but I can't think of a reason to prefer casting truthiness to a boolean value outside of the sort of convoluted "compare truthinesses" case, above, except for readability if you reuse the value later, as in the q library example.

But even then, it's a information-lossy shortcut, and I'd argue you're better off evaluating truthiness each time.

React is the main reason we've started using the!! I only have to worry about the truthiness or falseyness of something, not what the underlying type is. It's just the logical NOT operator, twice - it's used to convert something to boolean, e. Greg k 45 Why on earth would you do that? He's not comparing types; he's telling you what the results are, by writing assertions in his answer. It converts the suffix to a Boolean value. Paul McMillan 16k 7 47 Bill the Lizard k Prakash 5, 2 19 Season 1 The Deuce: Season 2 Fear the Walking Dead: Season 3 The Walking Dead: View All Photos 1.

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