Here, we collect all ActionScript 3 language features that are not present in ECMAScript 3 and document how we are going to simulate them.

General assumptions

We use

• ECMAScript 5 strict mode whenever possible
• ECMAScript 5 API (Object.create() etc.) that we know can be "polyfilled" in IE8 adequately
• a well-known module / dependency mechanism like CommonJS modules or AMD
• no other JavaScript library like jQuery, but "VanillaJS"

AS3 language features missing in ECMAScript 3

For every non-trivial language feature, we should introduce a dedicated child page.

Notation:

• $0, $1, ... are auxiliary variables generated by the compiler, chosen to avoid name-clashes with any other identifiers in scope.
• exp1, exp2, ... (italics) are arbitrary complex ActionScript expressions

Classes

Like most JavaScript frameworks, we simulate classes by a constructor function and its prototype chain.
The ES5 API Object.create(prototype,propertyDescriptors) serves this task very well.
While members are properties of the constructor function prototype, static members are properties of the constructor function itself.
Non-public members are discussed in a dedicated sub-section.

Class structure

The translation pattern is a follows:

public class Foo extends Bar {
  // constructor
  public function Foo(/*constructor parameters*/) {
    // constructor code
  }
  // member declarations
  ...
  // static member declarations
  ...
}

becomes

function Foo(/*constructor parameters*/) {
  // constructor code
}
Object.defineProperties(Foo, {
  ... // rewritten static member declarations
});
Foo.prototype = Object.create(Bar.prototype, {
  constructor: { value: Foo },
  ... // rewritten member declarations
});

The constructor property has to be defined explicitly, as otherwise, a subclass would inherit that property from its superclass, resulting in the subclass having the same constructor as its superclass.

Members and visibility (public, protected, internal, private)

Only public and protected members should be stored as properties of the constructor function / its prototype as-is. For protected members, access rights are checked by the compiler.
For private and internal members, we have to avoid name-clashes. There are different cases:

• Private methods (non-static as well as static ones) and private static fields are declared in new scope, shared by all class members. Thus, they are visible for every method, but not from outside. Non-static private method calls have to be rewritten to run with the correct this.
• Private non-static fields are renamed: they are post-fixed by $ plus the inheritance level of the declaring class, so that they cannot name-clash with fields of the same name, declared in a subclass or superclass.
• Internal members are post-fixed by $ plus the full package name (with . replaced by _), so that they cannot name-clash with members of the same name, declared in a subclass or superclass residing in another package.

Interfaces plus "as" and "is" operator

TODO

Packages

In ActionScript, packages do not exist at run-time, instead they lead to class and interface declarations having a fully qualified name to avoid name clashes.
To make a JavaScript expression referencing a fully qualified class resemble the ActionScript syntax, packages are often simulated by nested JavaScript objects, for example a class org.apache.flex.test.Foo would be assigned to a package object and then be used like so:

if (!org) org = {};
if (!org.apache) org.apache = {};
if (!org.apache.flex) org.apache.flex = {};
if (!org.apache.flex.test) org.apache.flex.test = {};
org.apache.flex.test.Foo = Foo;

var foo = new org.apache.flex.test.Foo();

(Of course, the code to create a nested package would be moved to a helper function.)
Looking at how classes are usually used in ActionScript, it turns out that they have to be imported (in contrast to Java, even if they are used by their fully-qualified names in the code!) and are only used by their fully-qualified names in the rare case of local name clashes.
Also, nested object access to fully qualified identifiers is not very efficient.
In JavaScript, there is a better solution to accessing namespaced identifiers, namely modules. There are essentially two types of module definitions: synchronous (CommonJS-style) versus asynchronous module definitions / require-calls. While synchronous requires come from the server world (e.g. NodeJS), asynchronous requires better fit realities of module loading in the browser.
When a module with a fully qualified name / path is required (loaded), it can be assigned to a local variable or a callback parameter, so that there is no need to use the fully qualified name.
Only when "exporting" ActionScript classes or other identifiers for direct use from JavaScript, it may make sense to apply the pattern "packages as global nested objects".

So the current suggestion for packages is to define a class that is inside a package as a module with the fully qualified name of that class.

Consider this class

org/apache/flex/test/Foo.as:

package org.apache.flex.test {
public class Foo {
  ...
}
}

and another class in another package using it:

org/apache/flex/test2/Baz.as:

package org.apache.flex.test2 {
import org.apache.flex.test.Foo;
public class Baz {
  ... // use Foo...
}
}

In JavaScript AMD notation, these classes would look like so:

org/apache/flex/test/Foo.js:

define("org/apache/flex/test/Foo", function() {
  function Foo() { ... }
  ...
  return Foo;
});

org/apache/flex/test2/Baz.js:

define("org/apache/flex/test2/Baz", ["org/apache/flex/test/Foo"], function(Foo) {
  function Baz() { ... }
  ... // use Foo...
  return Baz;
});

The first argument to define can be left out when the definition is the only one contained in the file, then the module name is derived from the file name.

Parameter default values

TODO

Rest (...) parameter

TODO

Bound methods

TODO

"this" is always in scope

TODO

Statements

TODO

for each

TODO

Operators

TODO

&&=

TODO

||=

TODO