Creating/populating Javascript custom object

Question

I've created an ashx page which is going to serve me an XML document full of basic user information. I'm not sure which is the best way to go about creating and populating my custom javascript object. I've seen them created in two ways:

function User() {
   this.Id;
   this.FirstName;
   this.LastName;
   this.Title;
}

and

var User2 = {
   Id: null,
   FirstName: null,
   LastName: null,
   Title: null
}

I could populate each of these by doing something like:

//first object
User.Id = 1

//second object
User2.FirstName = 'John'

Is one method of creating the object better than the other?

Edit: A year and a half later I saw that this question got the popular badge, so I just wanted to mention that today I am using Crockford's module pattern.

Answer 1

You cannot access the first object's properties without instantiation, i.e. using the new keyword:

 var myUser = new User() ;
 document.write(myUser.id) ;

The second object is an object literal which is accessible without instantiation as it is already instantiated when parsed.

The difference comes into play if you want use prototypical inheritance to create a new object on basis of the old one. Writing an object literal is probably easier to understand and the more appropriate pattern if you have a rather compact code base. However, prototyping comes in handy if you want to create a new object by augmenting an existing object with another object without having to rewrite the object getting augmented:

ipUser.prototype = User ;
ipUser.ip =  "128.0.0.1" ;

In your case this difference might not seem striking, but you have to imagine how much redundant code you would get if you would create another object literal for every meager addition to the original object.

Look into the Mozilla Developer Center's page on JavaScript Objects if you have additional questions, it's outlined pretty well: https://developer.mozilla.org/en/Core_JavaScript_1.5_Reference:Global_Objects:Object .

Hth

Answer 2

You don't have to create an object with empty values first. JavaScript doesn't need place holders and can add properties and methods dynamically at any time. That is, this would suffice as well:

var User2 = {};
User2.Id = 1;
User2.FirstName = 'John';
//...Etc.

If you're just concerned about storing data I'd use this form (the object literal, ie. your second method).

Update: You could also make things a bit easier and create a function that creates user objects for you:

function createUser(id, firstname, lastname, title) {
    return {
        Id: id,
        FirstName: firstname,
        LastName: lastname,
        Title: title
    };
}
var User2 = createUser(1, 'John', 'Smith', 'Manager');