What does this Javascript syntax mean?

Question

I am working with ROS structure and facing some problem with debugging, and I found this snippet:

ros.visualization.GLNode = Class.extend({
    init: function () {
        this.meshGroupIDs = [ ];
        this.matrix       = sglIdentityM4();
        this.children     = [ ];
    },
});

What do these lines mean?

ros.visualization.GLnode :: Is it trying to load some files? What does this . represent?

Class.extend:: What does this do? Is it trying to extend the class, and if so then which one? In my way it will be GLnode.

this.matrix:: I always have doubts when I see this command what does it do?

init : function() :: What does this function mean in js? Is it a keyword? If not then why does everyone use it in onloadbody or window.onload()?

I think one of the best way to answer all of your questions would be to go through a primer on javascript to learn of the language features and usage.. this is one I love: developer.mozilla.org/en/a_re-introduction_to_javascript — Quintin Robinson
– Quintin Robinson, Commented Jul 7, 2011 at 19:43
I found something useful which may help you too code.google.com/p/oopsjs tinyway.wordpress.com/2010/09/05/… developer.mozilla.org/en/… robertnyman.com/2008/10/09/… — zod
– zod, Commented Jul 7, 2011 at 19:51

jfriend00 · Accepted Answer · 2011-07-07 19:49:50Z

ros.visualization.GLNode = Class.extend({

This is the definition of a new class (or type of object) using some sort of javascript library that has the function Class.extend() in it.

init: function () {

This is the definition of an attribute on the new class called "init". The type of the attribute is "function" which means it works like a method and can be called.

this.meshGroupIDs = [ ];

This is the definition of a member variable on an object of this class and initializes it to an empty array.

this.matrix = sglIdentityM4();

This is the definition of another member variable on an object of thisclass and initializes it to the return value from the sglIdentityM4() function call.

this.children = [ ];

This is the definition of a member variable on an object of this class and initializes it to an empty array.

"this" is a reference to the local object and is how you address instance variables of the object you are in.

Matthew Crumley · Accepted Answer · 2011-07-07 20:02:43Z

ros.visualization.GLNode

The . operator lets you access object properties, so ros.visualization.GLNode = ... means that there is an object called ros, with a visualization property, and you are assigning something to the GLNode property of that object.

Class.extend

Class.extend isn't a built-in part of JavaScript, so without knowing how it was defined, it's impossible to know exactly what it's doing. I can guess from the name and how it's used that it creates a new "class" or extends an existing one. The code is passing an object literal (the { ... } part) that tells it how to define the class.

this.matrix

this is the context that a function was called in. When you call a function on an object (in this case, someGLNodeObject.init()), this gets set to that object. So this.matrix = sglIdentityM4(); is setting the matrix property of the current object.

init: function()

Defines a function in the object literal called init. The Class.extend function presumably uses the init function like a constructor to initialize new GLnode instances.

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What does this Javascript syntax mean?

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