Variable declaration methods in Javascript

Question

I have seen the following code in a website... what does this mean?.Can i declare variables in the format variableName : value instead of variableName = value.

if (!window.Node){
       var Node =
      {
        ELEMENT_NODE                :  1,
        ATTRIBUTE_NODE              :  2,
        TEXT_NODE                   :  3,
        CDATA_SECTION_NODE          :  4,
        ENTITY_REFERENCE_NODE       :  5,
        ENTITY_NODE                 :  6,
        PROCESSING_INSTRUCTION_NODE :  7,
        COMMENT_NODE                :  8,
        DOCUMENT_NODE               :  9,
        DOCUMENT_TYPE_NODE          : 10,
        DOCUMENT_FRAGMENT_NODE      : 11,
        NOTATION_NODE               : 12
    };
}

Answer 1

Those are object properties in an object litteral.

Empty object litteral:

var obj = {};

With properties:

var obj = {
    foo: "bar",
    test: 123
};

You can then access properties this way:

alert(obj.foo);

Note that this notation only works inside of object litterals. If you want to set a property from outside, also use the dot notation:

obj.foo = "hi";

Answer 2

its an Object Literal.

Follow some tutorials,you'll understand more.. Here's a small tutorial

http://www.dyn-web.com/tutorials/obj_lit.php

Answer 3

The line:

if (!window.Node){

Means, if the Node variable doesn't yet exist, then do the following:

Node is initialized to an object literal (basically a hash table). For example:

Node['ENTITY_NODE'] is equal to 6.

This can also be expressed as Node.ENTITY_NODE as well.

Answer 4

The variableName: value format is what is used to statically declare javascript properties of an object. In your example, Node is a new object and they are declaring 12 properties for it. You can do that too for property declarations, but property declarations are not exactly the same thing as a variable declaration.

What this code means is: "if window.Node doesn't already exist, then declare it as an object with these 12 properties".

It can then be accessed like this:

Node.ELEMENT_NODE == 1

The actual purpose of this code is to make sure that these node values are declared once and only once in a given web app so that they can be used by relevant code using meaningful symbol names rather than just comparing to a number.