!function () {}();
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.