What are the key differences between JavaScript and ActionScript 3?
First of all ActionScript 3 and JavaScript are both defined in ECMA-262 so they have a lot in common. Both languages feature prototype inheritance for instance. It is however not correct that ActionScript fully implements ES4.
ActionScript implements a couple of features that are not defined in ECMA-262 and some -- but definitely not all -- of ES4.
So what does AS3 add to ECMA-262? Those are also the differences to JavaScript:
- Dynamically and statically typed code
- Packages, Classes and Interfaces
- Standard OO inheritance model (not prototype based, statically typed)
- uint and int datatype
- E4X (ECMA-357)
- Type-safe conditional compilation (ES4)
- Vector.<T> datatype (ES4)
Maybe I have forgotten some features. I am not sure if XML, XMLList etc. are already defined in 262 or came with 357.
The key difference however is the standard library. JavaScript comes with a couple of predefined classes like DOMElement and browser dependent additions. ActionScript has a fairly large standard library with features like video streaming and is consistent over all platforms.
I've been programming in both ActionScript and Javascript, and from a less-technical standpoint, I see two main differences.
1) JavaScript is more powerful. You are allowed to do much more with the language because it does not have a "compiler" or types. There are some great frameworks out there like ExtJS and jQuery that try and simplify things for you, but even with them, you are really allowed to do an amazing amount of damage if you want to.
2) ActionScript is much more confining and hence, much easier to maintain. Adobe did a lot of work to keep you out of the difficult parts of ECMAScript. ECMAScript Objects, prototypal inheritance, and closures are three concepts that you really don't need to understand to program in ActionScript. You just need to understand how to use Adobe's "Class" object.
For simple uses, I prefer JavaScript. However, once the project gets large, it depends who you are coding for. If I had a team of 5 developers programming at a scrappy start-up, I'd choose JavaScript in a heartbeat. However, in the girth of a large corporation, or academia, you might be safer relying on Adobe's platform.
Hope that helps.