What is the usage of !! in JavaScript

Question

Possible Duplicate:
What is the !! (not not) operator in JavaScript?

I just came across the code that uses !!, which means logical not not to me.

  app.isArray = Array.isArray || function(object) {
    return !!(object && object.concat
              && object.unshift && !object.callee);
  };

What is the different between using !! and not using it?

(my guess is !! will convert the result to Boolean type. if my guess is correct, why is it so?)

@WesleyMurch thanks. I tried searching it, but did not fint it. Maybe I need to improve my search keywords. :) — Mifeng, Aug 01 '12 at 07:24
No it's hard to search for stuff like that. Only way I found it is through here: http://stackoverflow.com/questions/9549780/reference-what-does-this-symbol-mean-in-javascript (anyone feel free to cast reopen votes please!!!) — Wesley Murch, Aug 01 '12 at 07:25

score 5 · Accepted Answer · answered Aug 01 '12 at 07:22

5

!! Just inverts the content double into a boolean value. Here are some examples:

!! true === true
!! false === false

answered Aug 01 '12 at 07:22

rekire

1

Wesley Murch has already posted the link to a duplicate. Why post an answer then? – Ashwin Singh Aug 01 '12 at 07:23
5

I wrote the answer on my Mobile before I saw that comment. – rekire Aug 01 '12 at 07:25
thanks. Now I know that !(sth) will return boolean type false always (sth is true). !(!sth) will then convert the false to true. :) – Mifeng Aug 01 '12 at 07:26
2

Note that in your examples `!!` is redundant given the operands are already booleans... – nnnnnn Aug 01 '12 at 07:28
The duplicate does not answer OP's question Good job rekire providing answer. – user1260310 Dec 05 '12 at 12:23
I use it when there is a possibility of data value being null.. and the expected data type is boolean. !!null -> false – slopeofhope Sep 13 '19 at 20:13

1 Answers1