What does "!--" do in JavaScript?

Question

I have this piece of code (taken from this question):

var walk = function(dir, done) {     var results = [];      fs.readdir(dir, function(err, list) {         if (err)             return done(err);          var pending = list.length;          if (!pending)              return done(null, results);          list.forEach(function(file) {             file = path.resolve(dir, file);             fs.stat(file, function(err, stat) {                 if (stat && stat.isDirectory()) {                     walk(file, function(err, res) {                         results = results.concat(res);                          if (!--pending)                             done(null, results);                     });                 } else {                     results.push(file);                      if (!--pending)                          done(null, results);                 }             });         });     }); }; 

I'm trying to follow it, and I think I understand everything except for near the end where it says !--pending. In this context, what does that command do?

Edit: I appreciate all the further comments, but the question has been answered many times. Thanks anyway!

Answer 1

! inverts a value, and gives you the opposite boolean:

!true == false !false == true !1 == false !0 == true 

--[value] subtracts one (1) from a number, and then returns that number to be worked with:

var a = 1, b = 2; --a == 0 --b == 1 

So, !--pending subtracts one from pending, and then returns the opposite of its truthy/falsy value (whether or not it's 0).

pending = 2; !--pending == false  pending = 1; !--pending == true pending = 0; !--pending == false 

And yes, follow the ProTip. This may be a common idiom in other programming languages, but for most declarative JavaScript programming this looks quite alien.

Answer 2

That's not a special operator, it's 2 standard operators one after the other:

1. A prefix decrement (--)
2. A logical not (!)

This causes pending to be decremented and then tested to see if it's zero.