5

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?

Improve this question
5
  • 1
    possible duplicate of What is the !! (not not) operator in JavaScript? Commented Sep 2, 2013 at 13:08
  • 1
    possible duplicate of Double exclamation points? Commented Sep 2, 2013 at 13:09
  • 1
    And also check this javascriptturnsmeon.com/the-tilde-operator-in-javascript Commented Sep 2, 2013 at 13:10
  • 1
    As for what it is... it's a pointless syntax that saves 0 characters when typing and lowers legibility. varA.search(re)>-1 is it's 'brother' Commented Sep 2, 2013 at 13:14
  • I don't believe that this is a duplicate of other double-exclamation questions. !!~ 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). Commented Sep 2, 2013 at 13:16

3 Answers 3

Reset to default
11

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:

  • convert the return value to an integer
  • invert the bits of the integer
  • inspect the number for "truthiness" (zero or non-zero, false or true)
  • invert the boolean value
  • invert it again

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.

Improve this answer
Sign up to request clarification or add additional context in comments.

5 Comments

For reference, the !! 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.
@cHao yes, in this example it's not necessary.
@cHao In fact you could go further and say that the ~ is overkill, as you can just add +1 so the "no match" return value is falsy and the "match in first position" becomes truthy.
@Kolink: You could. But then again, "~x".length < "x+1".length. :)
But I've never once used ~ 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.
4

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))
Improve this answer

6 Comments

Your second example would return true for -1, giving a false positive
@Bojangles: I must be missing something. I'm not seeing the opportunity for a false positive here. Note that the second example uses .match rather than .search.
If varA.search() returns -1, the if() will satisfy, however no match was found, therefore a false positive
@Bojangles Like cHao said, the second example uses MATCH, not SEARCH.
Well crap, that's embarrassing... sorry :(
|
0

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

http://dreaminginjavascript.wordpress.com/2008/07/04/28/

Improve this answer

Comments

Start asking to get answers

Find the answer to your question by asking.

Ask question

Explore related questions

See similar questions with these tags.