TypeScript for ActionScript Developers

Introduction

The following covers what the TypeScript language is compared to ActionScript 1, 2, and 3 with as much context as possible and compares the syntax side by side. This article should help serve as a reference while you learn TypeScript. Also, these articles always help me learn while writing them and I like to have documentation I can refer to later.
You cannot really talk about ActionScript without talking about the Flash Player internals just like you can’t really talk about JavaScript for web development without also talking about browser internals. Thus, I’ve tried my best to make what the ActionScript equivalents are in the JavaScript browser world.

What is TypeScript?

TypeScript is awesome. It’s this new language from Microsoft that enhances JavaScript. Like Robert Penner said, it’s basically ActionScript 2. You get strong-typing at compile time, but your code is compiled to ActionScript 1/JavaScript at runtime and has no enforcement of types. It has a variety of features, but the only two that really matters are the optional strong-typing and the ability to work with existing JavaScript libraries in a strongly-typed way. There are more, but the only ones ActionScript developers will really care about are:

1. Optional Typing
2. Classes
3. Packages via Namespaces
4. supports private
5. modules
6. everything compiles to JavaScript
7. everything using classes/packages/modules compiles to RequireJS/AMD/CommonJS
8. You can code in normal JavaScript; TypeScript is optional; you can mix and match
9. all the TypeScript features for typing aren’t compiled into JavaScript (no enforcement) so it’s normal JavaScript that gets outputted

Again, there are some other cool features such as overloading, interfaces, statics, follows ECMAScript 6, allows you to “use” ECMAScript 6 now, open source, blah blah blah, etc. I’m only focusing on that ones that matter.
Here its creator, Anders Hejlsberg of Turbo Pascal, Delphi, and C# fame, does a 1 hour overview.

Why TypeScript?

If you want strong-typing in JavaScript, you’ll love TypeScript. Its typing is near identical to ActionScript 2 and 3.
If you are a server-side developer who prefers to render the client side, think Google Closure annotations are sufficient, unit tests who’s sole purpose is to catch spelling errors & type casting errors, or believe “loose typing” is a great feature of JavaScript, peace the eff out.
To quote Cliff Hall on a recent post defending languages that compile to JavaScript on Google+:

They let you apply useful abstractions like classes, interfaces, inheritance, encapsulation, and OOP design patterns. This allows you to build larger systems in a more maintainable way, that is more team-friendly.

Yep. For me, spelling mistakes and type issues that aren’t parsing related should be trivial issues, but instead have a major detrimental impact because of the current JavaScript development landscape and browser runtime issues (swallowed exceptions, misleading error messages, still not developer friendly compared to more mature runtimes, etc). TypeScript, at least in Visual Studio, will help alleviate the spelling & type casting issues, and if you do write unit tests, they’ll test more important things.

How? TypeScript Compiler

TypeScript the compiler is written in TypeScript. The compiler will output to JavaScript, optionally with header definitions. If you go to http://www.typescriptlang.org/ you can learn how to install and/or download what you need. I used the Node install. For Mac, this is nice because it makes it global so you can play in the Mac Terminal.
npm install -g typescript
If you want to compile some TypeScript to a JavaScript file in the same directory, on a command line, go: [...]
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