I am looking through some code that someone else have written and I noticed this strange javascript if syntax.. Basicly, it looks like this:
// This is understandable (but I dont know if it have relevance)
var re = new RegExp("^" + someVar + "_", "i");
// !!~ ??? What is this black magic?
if (!!~varA.search(re)) { ... }
This is one of those things that is hard to google.. Any Javascript gurues that can explain this?
Unary operators like that just need to be interpreted from right to left. ~ is the bitwise "not" operator, and ! is the boolean inverse. Thus, those three:
false or true)The ~ here is the trickiest. The "search" routine (I surmise) returns -1 when it doesn't find anything. The ~ operator turns -1 to 0, so the ~ allows one to interpret the "search" return value as true (non-zero) if the target is found, and false (zero) if not.
The subsequent application of ! — twice — forces the result to be a true boolean value. It's applied twice so that the true/false sense is maintained. edit Note that the forced conversion to boolean is not at all necessary in this particular code; the normal semantics of the if statement would work fine with just the result of the ~ operator.
!! is entirely unnecessary here. You'd only need it if you were storing the result for later and wanted to be sure it was a boolean. For immediate use in an if, it's overkill.
~ is overkill, as you can just add +1 so the "no match" return value is falsy and the "match in first position" becomes truthy."~x".length < "x+1".length. :)~ for any legitimate purpose, and I'm sure most other programmers don't use it either. Okay if you're going for minimalist, but if you're encouraging readability... Heh.Basically, .search returns the position at which it finds the result, or -1 if it doesn't match. Normal people would just write:
if( varA.search(re) > -1)
But personally I'd just use:
if( varA.match(re))
true for -1, giving a false positive.match rather than .search.varA.search() returns -1, the if() will satisfy, however no match was found, therefore a false positive:(In laymans terms
~
is doing -(N+1) and
!!
The first bang casts from the number to a Boolean, and the second undoes the logical not that was performed by the first bang.
Have a look at this website.
it has a few explanations
varA.search(re)>-1is it's 'brother'!!~is quite different from!!. I'll admit,!!~is new to me. It is cryptic and I think it warrants its own question. I'd always prefer a much more expressive approach:if(varA.search(re) >= 0).