Francis Cheng: Object Initializers and Fixtures for ECMAScript 4th Edition

A recently released ECMAScript 4 draft specification for object initializers includes significant enhancements to the expressive power of object initializers. This post focuses on the ability to designate specific properties as fixtures, which is not possible in ECMAScript 3rd edition or ActionScript 3.0.

You may have used object initializers in ActionScript 3.0 before. They provide an easy way to assign a literal value to a variable of type Object. For example, in ActionScript 3.0, you may see something like:

var myObj:Object = {name:"foo", color:"red"};
trace(myObj.name, myObj.color); // foo red

This creates an object called myObj with two dynamic properties, name and color. The properties are dynamic in that they are not defined as part of the Object class and can be deleted at runtime. For example, the following code deletes the name property:

// ActionScript 3.0
var myObj:Object = {name:"foo", color:"red"};
delete myObj.name;
trace(myObj.name, myObj.color); // undefined red

In ActionScript 3.0, if you want to ensure that the name property cannot be deleted at runtime, you have to define a class and make name a member, or "trait", of that class. In ActionScript 3.0, a trait is a class member that cannot be deleted at runtime. In ECMAScript 4, traits are called "fixtures", but you don't have to create a class definition to create a fixture. In fact, you can create fixtures with object initializers. For example, let's say you want to make the name property a fixture, so that runtime code cannot delete the property. In ES4, you'll be able to do this:

// ECMAScript 4 proposed
var myObj = {var name:"foo", color:"red");
delete myObj.name // error in strict mode, otherwise no effect

What if you want both name and color to be fixtures? All you have to do is place the "var" keyword before the curly braces. Doing so will make every property you name a fixture, like so:

// ECMAScript 4 proposed
var myObj = var {name:"foo", color:"red");
delete myObj.name; // error in strict mode, otherwise no effect
delete myObj.color; // error in strict mode, otherwise no effect

The fun doesn't stop there, though. Let's say you like creating objects on the fly like this, but that you'd want each object to have a unique ID number property that not only is a fixture, but also is a constant (i.e. read-only). All you have to do is use the keyword const instead of var:

// ECMAScript 4 proposed
var myObj = {const id:1, name:"foo", color:"red");
delete myObj.id; // error in strict mode, otherwise no effect
myObj.id = 2;    // error in strict mode, otherwise no effect

You can also use the const keyword before the curly braces if you want to make all of the properties you name read-only fixtures.

// ECMAScript 4 proposed
var myObj = const {id:1, name:"foo", color:"red");
myObj.id = 2;  // error in strict mode, otherwise no effect
myObj.name = "other";    // error in strict mode, otherwise no effect

Here's a list of relevant links:

Comments

Yeah Thanks for the information about ECMA4 :) It seems to be very powerful. Does that will impact ActionScript performances ?

Posted by: Cédric Tabin | April 16, 2008 12:05 PM

Francis,
Will we see a performance gain by leveraging fixtures in the new language? One of the comments from Jim Corbett and Lee Thomason's presentation on the AVM at MAX mentioned that using dynamic objects requires the AVM to lookup the property to determine if it exists and then what the value is. By leveraging fixtures will this help streamline the VM and treat it more like a sealed class or is this more to help implement structure with dynamic objects?

Posted by: James Polanco | April 16, 2008 12:12 PM

I think that you should see similar performance improvements whether you create fixtures with object initializers or with classes. As I understand it, what's happening behind the scenes in my examples above is that anonymous subclasses of Object are created. That would suggest to me that fixtures are fixtures, regardless of how they are created.

Posted by: Francis Cheng | April 16, 2008 01:33 PM

Thanks for keeping us up to date an some of the neat proposals for the ES4 spec. I'm looking forward to seeing how this affects actionscript, but for now it gives me some interesting food for thought.

Posted by: Daryl Ducharme | April 16, 2008 01:41 PM

I definitely like the "const" on the object. That's pretty sweet (not sure of applicability yet but looks potentially useful).

Posted by: John C. Bland II | April 16, 2008 07:25 PM

Francis Cheng

ActionScript, ECMAScript and the Flash Platform

Object Initializers and Fixtures for ECMAScript 4th Edition

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