Question

I’m a comer at JavaScript, and just learning some basic stuff (function, variable, etc) and I don’t know so much about complex structure in JavaScript.

When I read the source code of one of the web applications, there is a big point that I cannot understand. Here’s the code :

Helper.using('py.Figures' , function (ns) {
    ns.Point = function (params) {
        ns.Figure.call(this, params);
        this.setType('Point');

        this.visual(new Kinetic.Circle({
            radius:5, strokeWidth:2,
            fill:'red', stroke:'black',
            draggable:false
        }));

        this.getX = function () {
        };
        this.getY = function () {
        };
        this.getPosition = function () {
            return {
                x:this.getX(), y:this.getY()
            }
        }

        ns.Point.distance = function (p1, p2) { // some code }

        ns.MidPoint = function (params)  { // some code }       
};

1) First point that I don’t know in above code is the first line declaration :

Helper.using('py.Figures' , function (ns) {... } );

Many files in this document use this structure. In this document, there is a file name Helper, but when I search in this document, I don’t see some thing like py.Figures (by using Control + F). So, what does it really points to?

Above line looks something like a function, so what does function(ns) mean in this, it looks like parameter, but I don’t think so.

2) The second point I don’t know is :

ns.Point, ns.Point.distance, ns.MidPoint look like methoda. So, function(ns) is a class, right ? And if ns.Point is a method, why in this method, there are other methods like getX and getY and it makes me feel that those look like a class too.

Sorry if my question is silly, but this code looks strange to me, and I don’t see anything familar with some language I have learnt (Java, C#), or another scripting language too (Python)

Thanks.

Answer 1

1) That function (ns) {... } is indeed an argument. The function is parsed, and the pointer to the function is passed to the function Helper.using. You won’t see these thing in Java, but I think it’s one of the awesome points of Javascript.

It’s the usual way to define callbacks, and that argument function looks like a callback indeed.

2) No, ns is a class object, or better, is generically an Object. It doesn’t need to be an instance of a class. Then the callback function defines the method Point on the object ns. In the body of the method, it defines (or maybe redefines) the methods getX, getY and getPosition, because this refers to the object ns.

On a second look, ns.Point is indeed a function, but it may serve as a class definition, so in other parts of the code you may notice something like new ns.Point(...). That’s how “classes” work in Javascript.