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What does ‘!!’ mean/is used for?

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May 16.2011

Hi

I’m reading a book about HTML5 and I have found the following expression:

function supports_canvas() {
return !!document.createElement(‘canvas’).getContext;
}

Is ‘!!’ a double negation? In this case, why is that used? Is it not the same that

function supports_canvas() {
return document.createElement(‘canvas’).getContext;
}

I have seen this ‘!!’ more and more in JS code.

Thanks a lot

Isi

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JavaScript

10 Comments(s) ↴

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@KorMay 16.2011 — The javascript double negative !! is used to turn any data type (null, undefined, objects) to a Boolean.

The coder wanted, in your case, to make sure that the returned value is a Boolean.
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@islegmarauthorMay 16.2011 — Aha! Now I see it :-) Thanks a lot.

Isi
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@KorMay 16.2011 — In fact JavaScript has a native method: [B]Boolean([I]variable[/I])[/B], to do that. But either the coders don't know about Boolean(), or they prefer a little bit shorter (and faster) notation.
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@rnd_meMay 16.2011 — it's redundant 99% of the time; anything that is truish with "!!" is truish without it.
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@KorMay 16.2011 — it's redundant 99% of the time; anything that is truish with "!!" is truish without it.[/QUOTE]
Agree. But sometimes you might need straight the Boolean evaluation. Like in setting dynamically a certain value for a property. Like checked, readOnly... things like that ?
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@rnd_meMay 16.2011 — Agree. But sometimes you might need straight the Boolean evaluation. Like in setting dynamically a certain value for a property. Like checked, readOnly... things like that ?[/QUOTE]

even in those cases, "!!" still redundant and not needed.

firebug examples for this page's quick-reply box:
[CODE]
elm = $("vB_Editor_QR_textarea");
elm.readOnly="false"; //is actually==true



elm = $("qr_quickreply");
elm.checked= alert; // fn/obj is truish...
setTimeout(function(){elm.checked = 5 * 0;}, 1500); //zero is falseish.
[/CODE]
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@KorMay 16.2011 — I know. But as I said, it just a matter of changing the type of the variable.
<i>
</i>var x='a';
alert(typeof(x))
alert(typeof(!!x))

Not of much use, I reckon. A defined variable is always taken as a Boolean true, if necessary, and an [B]undefined[/B] or [B]null [/B]as a Boolean true. I have just explained the OP what that double negative was for.
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@rnd_meMay 16.2011 — typeof is an operator, not a function.
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@KorMay 16.2011 — typeof is an operator, not a function.[/QUOTE]
Who said it is not an operator? But it is a method as well. It is not my fault that it works as a method. ? Maybe Crockford's:

http://javascript.crockford.com/remedial.html
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@rnd_meMay 16.2011 — Who said it is not an operator? But it is a method as well. It is not my fault that it works as a method. ? maybe Crockford's:

http://javascript.crockford.com/remedial.html[/QUOTE]

extra parens are ignored, just like those around return values...

in the interest of not confusing noobs, as well as being a know-it-all, i just thought id' point it out. ?
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