Can I get some clarification on why I would want to use this?
myVar = !!someOtherVar;
+8
A:
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
2010-09-22 18:40:29
... or is it not not unrecommended?
STW
2010-09-22 18:41:53
There's no type casting in Javascript (note the "Javascript" tag), so !! is the best way to explicitly create a boolean value from the "truthiness" of an expression.
Pointy
2010-09-22 18:47:41
@STW: http://www.youtube.com/watch?v=0QOya9-lwQk (see 1:44 and 8:38)
BoltClock
2010-09-22 18:50:17
You can type cast in JS using `Boolean(someOtherVar)`
MooGoo
2010-09-22 18:50:29
@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
2010-09-22 18:58:37
@MooGoo -- thanks for my "how didn't I know that exists?" moment of the day
STW
2010-09-22 19:01:39
@MooGoo that is **not** a "type cast" - that's an object construction, and not the same thing at all.
Pointy
2010-09-22 19:08:16
@STW: yes, I do.
BoltClock
2010-09-22 19:12:23
@Pointy: in which situations does the difference between `Boolean()` and `!!` matter?
chelmertz
2010-09-22 19:16:31
@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
2010-09-22 19:28:36
@chimetz, @MooGoo I personally still wouldn't use the term "type cast", but I guess that's just a matter of preference.
Pointy
2010-09-22 19:29:11
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
2010-09-22 19:44:59
+4
A:
(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
2010-09-22 18:41:23
+2
A:
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
2010-09-22 18:41:29
+5
A:
It's a double negation, but it also works for type casting. !somevar will return a boolean (true, if somevar is "truthy" and false if it is "falsey", as per Crockford's lectures). So, !!somevar will be not(bool) and hence it will be boolean.
naivists
2010-09-22 18:41:52