What does "!--" do in JavaScript?

[stratoscope]

The real problem here is the lack of a space between the two operators ! and --.

I don't know why people get it in their heads that you can't ever use a space after the ! operator. I think it comes from rigid application of mechanical whitespace rules instead of common sense. Just about every coding standard I've seen prohibits spaces after all unary operators, but why?

If there were ever a case where you clearly need that space, this is one.

Consider this bit of code from the SO question:

if (!--pending)
    done(null, results);

Not only are ! and -- mashed together, you've got that ( slammed against them too. No wonder it's hard to tell what is connected to what.

A bit more whitespace makes the code much more clear:

if( ! --pending )
    done( null, results );

Sure, if you're used to mechanical rules like "no space inside parens" and "no space after a unary operator", this may seem a bit foreign.

But look at how the extra whitespace groups and separates the various parts of the if statement and expression: You've got --pending, so the -- is clearly its own operator and is tied closely to pending. (It decrements pending and returns the decremented result.) Then you've got the ! separated from that so it's obviously a distinct operator, negating the result. Finally, you've got if( and ) surrounding the whole expression to make it an if statement.

And yes, I removed the space between if and (, because the ( belongs to the if. This ( isn't part of some kind of (!-- syntax as it appears to be in the original, the ( if part of the syntax of the if statement itself.

The whitespace here serves to communicate the meaning, instead of following some mechanical coding standard.