Very straight-forward, so the question is why
(function(){ console.log('a'); }()); // 'a'
or
(function(){ console.log('a'); })(); // 'a'
work, but
function(){ console.log('a'); }() // SyntaxError: Unexpected token (
gives an error? What's the meaning behind this behavior? Or what's the purpose of those extra parentheses?
The extra parentheses convert the function into a function expression instead of a function declaration.
Only a function expression can be invoked immediately with the trailing ().
There is no clear explanation for that. JavaScript simply can't parse function(){ console.log('a'); }() successfully. This also happens with other examples, like this one:
1.toString(); // Syntax error
(1).toString(); // Works
An (working) alternative to your code would be (note the ! at the beginning):
!function(){ console.log('a'); }()
