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What is the !! (not not) operator in JavaScript?

I have encountered this piece of code:

function printStackTrace(options) { options = options || {guess: true}; var ex = options.e || null, guess = !!options.guess; var p = new printStackTrace.implementation(), result = p.run(ex); return (guess) ? p.guessAnonymousFunctions(result) : result; }

And I couldn't help to wonder why the double negation? And is there an alternative way to achieve the same effect?

(The code is from https://github.com/eriwen/javascript-stacktrace/blob/master/stacktrace.js.)

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It casts to boolean. The first ! negates it once, converting values like so:

  • undefined to true
  • null to true
  • +0 to true
  • -0 to true
  • '' to true
  • NaN to true
  • false to true
  • All other expressions to false

Then the other ! negates it again. A concise cast to boolean, exactly equivalent to ToBoolean simply because ! is defined as its negation. It’s unnecessary here, though, because it’s only used as the condition of the conditional operator, which will determine truthiness in the same way.

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7 Comments

@gdoron: 1 / 0 === Infinity; 1 / -0 === -Infinity
if(guess) is the same as if(!!guess)? For JS, what case would you need to do this for other than display purposes (showing that a variable is true or false)? It seems like an unneeded step, if you're only using it within a conditional expression or an IF statement, since the object's truthiness will always be evaluated. Is that accurate?
@JoePC: Yes, you’re correct in saying that it’s an unneeded step if you’re only using it within a conditional expression or an if statement.
@Ry- Thank you! I came across a good case for it after I posted that. When doing a return from a function and needing a boolean type to be returned and not the object itself, such as: return !!guess; I can see why the earlier usage is not totally necessary. (guess = !!option.guess)
I think this answer would be improved by including the above comments about when it is useful and when it isn't.
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var x = "somevalue" var isNotEmpty = !!x.length;

Let's break it to pieces:

x.length // 9 !x.length // false !!x.length // true

So it's used to convert a "truethy" "falsy" value to a Boolean.

The following values are equivalent to false in conditional statements:

  • false
  • null
  • undefined
  • The empty string "" (\ '')
  • The number 0
  • The number NaN

All other values are equivalent to true.

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35

Double-negation turns a "truthy" or "falsy" value into a Boolean value, true or false.

Most are familiar with using truthiness as a test:

if (options.guess) { // Runs if 'options.guess' is truthy, }

But that does not necessarily mean:

options.guess === true // Could be, could be not

If you need to force a "truthy" value to a true boolean value, !! is a convenient way to do that:

!!options.guess === true // Always true if 'options.guess' is truthy
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This answer made it clear. I had this same doubt between truthy and boolean value

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