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In someone else's code I found this way to test the existence of a variable : 

if(!!variable) { //Do something } else { //Something else }

Is there a reason to test with if(!!variable) instead of if(variable) ? Is it a good practice?

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  • 1
    Why, indeed? Seems less readable to me. Can't wait to see what others say. Commented Jun 30, 2016 at 9:20
  • Only use of !! i can think of is to convert something to bool. E.g. var i = !!0; console.log(i); //false (instead of 0). Commented Jun 30, 2016 at 9:20
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    In this particular case it doesn't make any difference, I think. But it's useful when you want to convert an arbitrary value into a boolean (when you later save it to the database, or something) Commented Jun 30, 2016 at 9:21
  • if converts always to boolean. Commented Jun 30, 2016 at 9:22
  • Some use it as explicit coercion instead of Boolean. Some find it more readable to explicitly tell they want to evaluate a boolean value. Commented Jun 30, 2016 at 9:28

4 Answers 4

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Is there a reason to test with if(!!variable) instead of if(variable)?

No.

Using if() will coerce the condition to a boolean anyway, there's no need to manually do it. Using this sort of syntax makes sense when you want to guarantee you've got a boolean elsewhere, for example if you wanted to log the truthiness of the expression:

console.log(!!variable)

This will ensure that you get either true or false logged, rather than the original value of the variable. But inside an if? It makes no difference.

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2 Comments

It's also useful along with other logical ops: 0 && true yields 0 for instance. If you expect a boolean value, try: !!0 && true
@LUH3417 Indeed. But both of those will behave the same way inside an if(), which is what the question was asking.
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No, in the case of a if, their is usually no reason to write that in JS.

I would not call it bad practice, since it can often come from programmer coming from strongly typed language, and is a common quick way to do the conversion to bool in thoses language.

It can also be used to emphasise the importance of the truthiness of the value in a long if, and show the programmer didn t forget to put the condition.

TL:DR: As for a lot of things: it may not be good practice, but it only become bad practice if your team is not used to that practice, either train them, document, or leave it.

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Double negation is a bit of a hacky way (IMHO) to convert a variable to bool. using ! on variable coerces it to a bool and negates it, and using it again convert it back to the 'correct' value. So !!0 === false

As mentioned by James there is no reason to do this inside an if statement. The only reason really to do this would be to coerce to a bool for logging purposes or similar so the value logged is true or false rather than 0, an empty string etc

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5 Comments

What is the difference between if(0) and if(!!0)
@SergioTulentsev There is no difference really because if coerces the value anyway.
This was my attempt at hinting that your answer doesn't answer the actual question.
@SergioTulentsev Fair enough. I misunderstood the question, but have tried to expand a bit (for what its worth)
I see that and I retracted my downvote. :)
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Double ! (!!) is not an operator. It's just ! twice - double negation.

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But why use it specifically here? It's not like it's being assigned to a variable that you want to guarantee is a bool.
How does this answer the question? JFYI, the question isn't "what is this !! operator and what does it do?".

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