create local variables accessible to all the prototype functions

Question

I'm trying to use prototype to add functions to an object, I thought I understand the whole concept, so here's what I did:

function ImgContainer() {
    var current_image = 1;
}

ImgContainer.prototype = {
    init: function() {
        //initialize
    },
    scrollLeft: function(){
        //scroll left
    }
}

var imgContainer = new ImgContainer();

I assume I can access current_image in both init and scrollLeft, but I'm getting Uncaught ReferenceError: current_image is not defined.

What should I do to have a variable that's accessible in both init and scrollLeft function?

Answer 1

You would add it as a property of the instantiated objects:

function ImgContainer() {
    this.current_image = 1;
}

Then access the property in the functions:

ImgContainer.prototype = {
    init: function() {
        alert(this.current_image);
    },
    scrollLeft: function(){
        //scroll left
    }
}

You can still use short lived variables inside methods to store stuff temporarily to get that method's job done. But you store the object's state in its properties.

Answer 2

You can't reach private members of an object via prototype methods outside the object because they are out of the scope. You should do it like this:

function ImgContainer() {
    var current_image = 1;
    this.init = function() {
        current_image = 'something_blahblah';
    }
}

var imgContainer = new ImgContainer();