IIFE or SEAF

Immediately-Invoked Function Expression or Self-Executing Anonymous Functions

So these are just anonymous functions that execute immediately.

//Example 1
(function(){
    console.log('Hello');
})()
//Example 2
//same as
(function(){
    console.log('Hello');
}())

//Example 3
//this does not work, fails with SyntaxError: function statement requires a name
function(){
    console.log('Hello');
}()

//Example 4
//this fails with SyntaxError: syntax error
function test(){
    console.log('Hello');
}()

Example 3 did not work because the parser thinks you are trying to define a function which does not have a name so it fails. Example 4 has a name but that fails as well, because a function definition is a statement and the trailing () that is suppose to invoke the function acts just as a grouper.

The ( ) that surrounds the function makes the parser parse the function as an expresssion and so allows it to be executed.

Whats the point?

Since Javascript only has function scoping and closures, this is a very useful pattern for encapsulating modules and making sure your namespace is not polluted with unwanted variables.

var timer = (function(){
  var count = 1;

  return {
    tick: function(){
      console.log(count++);
    }
   };

})()

timer.tick();// 1
timer.tick();// 2
timer.tick();// 3

console.log(timer);// Object { tick=function()}
console.log(count);//ReferenceError: count is not defined

Here the anonymous function returns an object that has a function that references a variable with in the anonymous function. As you can see the count is not accessible from outside the function.

Private Static Variables

In fact you can use this pattern to create static private variables. So creating multiple objects that all share one variable that is private to themselves. See below.

var SharedCounter = (
function(){
  var counter = 0;

  return function(){
    this.count = function(){
      console.log(counter++);
    }
  };
})();

var a = new SharedCounter();
var b = new SharedCounter();
var c = new SharedCounter();
a.count(); //0
b.count(); //1
c.count(); //2

console.log(a.counter) //undefined
console.log(counter)// ReferenceError: counter is not defined