How do I dynamically instantiate a class in javascript?

Question

I'm starting out with classes in Javascript and have hit a wall. I've looked all over for a tutorial that goes a little further than simply how to construct a class (usually Animal) then extend the class and have a Method do something (Dog alert('Bark');).

I have created a class that I want a user to be able to instantiate (is that the right word)? For example the first stage in my program is for the user to give the class a name, and then start to populate the various variables in the class. When they've done that they may do it again many times.

EDITED, FULL EXAMPLE OF CODE:

function Blind() {
    this.Type = null;
    this.Colour = null;
    this.Width = 500;
    this.Drop = 500;
    this.Price = 0;
}

function BlindAluminium() {
    Blind.call(this);
    this.Type = 'aluminium';
    this.SubType = null;
    this.StackHeight = 0;
}

That is the code that defines the object.

Here is the HTML that will allow the user to define a new object:

<form id="create_blind_form" name="create_blind_form" method="post" action="">
    <label for="blind_name">Blind Name: <input name="blind_name" id="blind_name" type="text" /></label>
    <input name="submit" type="submit" value="Submit" />
</form>

On submitting that I need a new object to be created, in pseudo-code, something like this:

var *blind_name_from_the_form* = new BlindAluminium();

Then later I hope to act on it like this:

*blind_name_from_the_form*.subType = '50mm';

Answer 1

The short answer is that you're going to have to use eval. If you want to call a function (or class) with a name known only at run-time, there's no way around it.

eval("var className = new MyObject();");

That being said, you may want to consider some alternatives. People will give you all sorts of reasons why eval is bad, which are true. But to me, going down this route is just going to be awkward to manage. Yes, dig into Crockford.

Is it possible to use Javascript's prototypical inheritance, instead of trying to use awkward classes? Given that classes mean nothing in Javascript, there's really no reason to use them if they don't fit the problem.

Answer 2

Maybe by storing the name inside another object?

var instances = {};
instances['theNameTheUserChose'] = new MyObject();

Answer 3

You are always going to have to have some sort of reference to your type. Lets say you create a constructor function (class) and call it MyObject. If I understand you correctly the use case is that your users should be able to do something like this instead:

var userObject = new SomethingElse(); // instead of new MyObject();

This is programming, so you are always going to have to be explicit. You could do something like this:

var types = {};
types["aType"] = MyObject;
var userObject = new types["aType"];

Note that even though you do like above, your users are going to have to know what the "aType" property of the "types" object contains. You can't really do something like this:

var userObject = guessTheClassName();

Hope this helps.