What does the exclamation mark do before the function?

Question

!function () {}();

Answer 1

! will negate (opposite) whatever you're expecting as a result, i.e if you have

var boy = true;
undefined
boy
true
!boy
false

when you call boy, your result will be true, but the moment you add the ! when calling boy, i.e !boy, your result will be false. Which in other words you mean NotBoy, but this time it's basically a boolean result, either true or false.

That's the same thing that happens to the !function () {}(); expression, running only function () {}(); will flag you an error, but add ! right in front of your function () {}(); expression, makes it the opposite of the function () {}(); which should return you true. Example can be seen below:

function () {}();
SyntaxError: function statement requires a name
!function () {}();
true