What is the (function() { } )() construct in JavaScript?

That is saying execute immediately.

so if I do:

var val = (function(){
     var a = 0;  // in the scope of this function
     return function(x){
         a += x;
         return a;
     };
})();

alert(val(10)); //10
alert(val(11)); //21

Fiddle: http://jsfiddle.net/maniator/LqvpQ/

Second Example:

var val = (function(){
     return 13 + 5;
})();

alert(val); //18