Can I get some clarification on why I would want to use this?
myVar = !!someOtherVar;
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12
In non-strictly typed languages, the ! operator converts a value to a boolean. Doing it twice would be equivalent to saying
myVar = (boolean)someOtherVar
Note that this is not recommended for code clarity.
phsource
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11... or is it not not unrecommended? – STW Sep 22 '10 at 18:41
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@STW: http://www.youtube.com/watch?v=0QOya9-lwQk (see 1:44 and 8:38) – BoltClock Sep 22 '10 at 18:50
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3You can type cast in JS using `Boolean(someOtherVar)` – MooGoo Sep 22 '10 at 18:50
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@BoltClock -- by the first minute I was having flashbacks to the South Park where Butters becomes a pimp; you know what I'm sayin? – STW Sep 22 '10 at 18:58
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@MooGoo -- thanks for my "how didn't I know that exists?" moment of the day – STW Sep 22 '10 at 19:01
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@MooGoo that is **not** a "type cast" - that's an object construction, and not the same thing at all. – Pointy Sep 22 '10 at 19:08
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@Pointy: in which situations does the difference between `Boolean()` and `!!` matter? – chelmertz Sep 22 '10 at 19:16
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@chlmertz, @MooGoo - you're right; when the `Boolean()` constructor is *called* (as opposed to being invoked with `new`), it seems to return either `true` or `false` - in other words, the constants, and not an object. Thus `true === Boolean(2)` is `true`, while `true === new Boolean(2)` is `false`. The MDC page pretty much says exactly this (and leaves one wondering just what use the objects might be). – Pointy Sep 22 '10 at 19:28
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@chimetz, @MooGoo I personally still wouldn't use the term "type cast", but I guess that's just a matter of preference. – Pointy Sep 22 '10 at 19:29
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It creates a bona-fide object that wraps the primitive value. As far as uses go, there's not many. You can add properties to the object (whereas with a primitive any added properties would be ignored), but it is useless in comparisons, because objects are only == to themselves in JS. I would say "type cast" is appropriate because it describes the behavior exactly, even if the implementation here is different than in most other languages (calling a function that returns a value vs special syntax) – MooGoo Sep 22 '10 at 19:44
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(Rewritten to clarify, simplify)
That statement performs a couple different actions:
myVar = // This portion is a regular assignment, it will store the value of the suffix
!!someOtherVar; // This portion is evaluated to a boolean result
The !!someOtherVar, I assume, is what you're really asking about. The answer is simple: it performs two logical NOT operations against the truthiness (a Javascript'ism) of someOtherVar.
In other words, if you understand the ! operator, this just combines two of them (!! isn't a different operator). By doing this it essentially returns the boolean evaluation of someOtherVar--in other words, it's a cast from whatever type someOtherVar is to boolean.
So... to walk through this, and pay attention to the result of myVar:
myVar = someOtherVar; // myVar will be whatever type someOtherVar is
myVar = !someOtherVar; // myVar will *always be boolean, but the inverse of someOtherVar's truthiness
myVar = !!someOtherVar; // myVar will *always be boolean, and be the equivalent of someOtherVar's truthiness
STW
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7
It's a double negation, but it also works for type casting. !somevar will return a boolean (true, if somevar is "falsey" and false if it is "truthy", as per Crockford's lectures). So, !!somevar will be not(bool) and hence it will be boolean.
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If you need to pass a boolean value to a function, or are anal about evaluating only booleans in conditional statements, that casts someOtherVar to a boolean for you by double-negating it.
BoltClock
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