anonymous javascript function call !function vs function [duplicate]

Question

How come

function(){ alert("test123");}()

produces SyntaxError: Unexpected token (

while

!function(){ alert("test123");}()

alerts "test123"

?

Answer 1

It's because by adding ! sign you convert the declaration into an expression and invoke it immediately. By enclosing your function into brackets you will make first example working without errors:

(function(){ alert("test123");})()

To make it clearer you can think about first expression as something like:

if (false || !function(){ return false; }())

---

And as @zerkms noticed there is a complete explanation of Immediately-invoking functions.