Question

Possible Duplicate:
What do parentheses surrounding a JavaScript object/function/class declaration mean?
What does this javascript syntax mean?
What does this “(function(){});”, a function inside brackets, mean in javascript?

In the below code the anonymous function is being executed.

var a= 1;
var b =2;           
(function() {  
    var b = 3;  
    a += b;  
})();  
document.write(a + " "+ b);

1) What does putting parenthesis around the function definition do?

2) What does putting () after the closing parenthesis do?

Answer 1

Putting the () around the function() { ... } makes it an expression vs. a statement. Because it’s an expression which produces a function value the () at the end invoke the produced function.

Consider the alternatives

function () { 
  var b = 3;
  a += b;
}();

This produces a syntax error as the () are essentially trying to invoke a statement.

(function () {
  var b = 3;
  a += b;
});

This produces a function object which is never invoked.