What does the exclamation mark do before the function?

Question

!function () {}();

Answer 1

Exclamation mark makes any function always return a boolean.
The final value is the negation of the value returned by the function.

!function bool() { return false; }() // true
!function bool() { return true; }() // false

Omitting ! in the above examples would be a SyntaxError.

function bool() { return true; }() // SyntaxError

However, a better way to achieve this would be:

(function bool() { return true; })() // true