What do these double parentheses do in JS? [duplicate]

Question

I'm reading the book secrets of the js ninja, and very often I saw code like this

(function(){
  something here;
  })();

Why do we need to enclose the function within parentheses and why do we add one more pair of parentheses after that?

Answer 1

This

(function(){
  alert('hello');
})();

although it is a function is it called automatically so you dont/can't call it manually

These can be useful for for loops like so

This will fail because i would be equal to 9 after 5 seconds

for(var i = 0; i < 10; i++) {
   window.setTimeout(function(){
      console.log(i);
   }, 5000)
}

So you could do this

for(var i = 0; i < 10; i++) {
   (function(a){
      window.setTimeout(function(){
         console.log(a);
      }, 5000)
   })(i);
}

Also good for creating a "private" scope like this

(function(){
   var test = 'hello';
   console.log( test ); // 'hello'
}());

   console.log( test ); // 'undefined'