It's up to you. All !! does is "cast" its argument to a Boolean.
jonchang
2009-12-14 12:07:36
+1
A:
! negates the result of whatever is on the right. So !! negates the negated value thus ending with whatever was originally on the right.
edit: the above is true if you have boolean values, results may vary for other types ...
edit2 to elaborate some more: !! is a "type cast" operator of sorts. if you have a boolean value on the right then nothing will happen. If you have something other then a boolean value on the right, then the first ! will convert whatever is on the right to the boolean "version" of that value, and the second ! will negate that value. Kinda like saying: return the true value of a non boolean value. Hope that makes sense :)
Jan Hančič
2009-12-14 12:07:44
not true. this is so only for booleans, which are not the question's subject.
Pavel Radzivilovsky
2009-12-14 12:08:59
It's true regardless of whatever the original type was.
jonchang
2009-12-14 12:09:02
That depends on whether whatever was on the right was a boolean value.
Dominic Rodger
2009-12-14 12:09:11
No, it doesn't. The result of a `!` or `!!` operation will always be a Boolean, regardless of its original argument.
jonchang
2009-12-14 12:11:34
in fact, no matter how many negations you tack together, the result is always boolean.
just somebody
2009-12-14 12:30:51
A:
And what if it is a string with value "undefined"?
I think !!(expression) is neat.
Pavel Radzivilovsky
2009-12-14 12:09:48
+3
A:
It's a common way to convert any return type to boolean (usually to avoid compilation warnings). And second: no, checking if type is "undefined" is mandatory anyway and "!!" can not cover it.
alemjerus
2009-12-14 12:10:03
What do you mean by "checking if type is "undefined" is mandatory anyway and "!!" can not cover it."?
Tim Down
2009-12-14 15:02:34
"!!" will only allow you to check if operand is zero of any king, but only "typeof()" can tell you if the operand is defined. "!!" for undefined operand will throw a error.
alemjerus
2009-12-14 15:20:11
OK, I see what you were getting at now. It's not always true that `!` (and hence `!!`) will throw an error for an undefined operand: for example, `!!window.bananas` will not throw an error while `!!bananas` will. But in general for testing if an object or property is undefined, `typeof` (an operator, not a function, so no parentheses required) is the way to go.
Tim Down
2009-12-14 16:33:50
+1
A:
var filter = !!(document.body.filters);
is NOT equivalent to
var filters = typeof document.body.filters != 'undefined'
!! merely checks if the operand is "truthy", i.e. whether it evaluates to true when used in a boolean expression. It has no relation to typeof. In general with host objects (such as document.body.filters) you are best off using typeof checks. The following article is good reading on this subject: http://peter.michaux.ca/articles/feature-detection-state-of-the-art-browser-scripting
Tim Down
2009-12-14 14:58:23