JavaScript syntax 101: here is a function declaration:
function foo() {}
Note that there’s no semicolon; this is just a function declaration. You would need an invocation, foo(), to actually run the function.
Now, when we add the seemingly innocuous exclamation mark: !function foo() {} it turns it into an expression. It is now a function expression.
The ! alone doesn’t invoke the function, of course, but we can now put () at the end: !function foo() {}(), which has higher precedence than ! and instantly calls the function.
function foo() {}() would be a syntax error because you can’t put arguments (()) right after a function declaration.
So what the author is doing is saving a byte per function expression; a more readable way of writing it would be this:
(function(){})();
Lastly, ! makes the expression return a boolean based on the return value of the function. Usually, an immediately invoked function expression (IIFE) doesn’t explicitly return anything, so its return value will be undefined, which leaves us with !undefined which is true. This boolean isn’t used.