Question

What type of function is the following and which is their best use?

var f = function (){
    var a = 0;
    return {
        f1 : function(){
        },
        f2 : function(param){
        }
    };
}();

I tried to convert it to:

var f = {
    a : 0,
    f1: function (){
    },
    f2: function (param){
    }
}

But seems does not works the same way.

Answer 1

It’s just a plain old function that is invoked immediately, and returns an object which is then referenced by f.

The functions referenced by object returned retain their ability to reference the a variable.

No code outside that immediately invoked function can reference a so it enjoys some protection since you control exactly what happens to a via the functions you exported.

This pattern is sometimes called a module pattern.

---

Regarding your updated question, it doesn’t work the same because a is now a publicly available property of the object.

For the functions to reference it, they’ll either do:

f.a;

or if the function will be called from the f object, they can do:

this.a;

Like this:

var f = {
    a : 0,
    f1: function (){
        alert( this.a );
    },
    f2: function (param){
        this.a = param;
    }
}

f.f2( 123 );

f.f1(); // alerts 123

But the key thing is that a is publicly available. Any code that has access to f can access f.a and therefore make changes to it without using your f1 and f2 functions.

That’s the beauty of the first code. You get to control exactly what happens to a.