ActionScript vs Typescript

Differences between ActionScript and Typescript

Warning: This document is a work-in-progress/undergoing review.

ActionScript has a different goal than Typescript

Typescript states their goals as providing compile type type safety and conforming to the Javascript spec supporting all features. Typescript does not provide runtime type safety or any tools beyond compile time types.

Royale’s goals are different. It provides a complete development environment. That includes the ActionScript language, debugging tools, a complete UI framework and both debug and deployment compiling.

While Royale give very high priority to performance, our philosophy is that besides compile time type safety, runtime type safety is important too. By providing certain runtime guarantees, a certain class of Javascript bugs disappear. (More details to follow…)

Testing equality

In Javascript, strict equality is recommended (i.e. ===) instead of “normal” equality (i.e. ==). That’s because Javascript automatically does a type coercion before comparing equality and can cause unexpected results.

Royale generally does not have these problems and bugs caused by using == are extremely rare. That’s because types are enforced at runtime time and uncertain values are automatically coerced when assigning the values. Normal equality has advantages and can be used for automatically evaluating certain types (such as XML). Therefore the general recommendation in ActionScript and Royale is to use == instead of ===.

The exception to this rule is if you are using untyped variables (i.e. *) and need to test for undefined to the exclusion of null. In that case you need foo.baz === undefined.

Strings and Numbers

Strings and numbers in Javascript can either be literal primitives (i.e. foo, 3.14) or classes (i.e. new String("foo"), new Number(3.14)). These are two distinct types in Javascript and in turn in Typescript. In Typescript, you would declare the former using string and number, while the latter would be String and Number. Trying to mix the two in Typescript will cause warnings.

ActionScript does not differentiate between the two types. Both types use the Uppercase notation, so you have String and Number, but no string or number 'foo' is String and new String("foo") is String both resolve to true in ActionScript. If you try to use strict equality on the two (i.e. ===), it will fail, but if you use the standard ActionScript practices, you don’t need to worry about whether strings and numbers are literals or not.

Arrays and Vectors

Typed arrays is an important piece in declaring types. ActionScript does not currently have typed Arrays. All Arrays are untyped with the exception of cases where implicit coercions are detected. For “typed arrays” in ActionScript you use Vectors. Vectors are similar to Arrays, but have some differences. Vectors has some features which can be enforced at runtime. See the page about Vectors for full details.


Dealing with XML in Javascript is difficult at best. Many people try to bash XML in defence of Javascript’s poor XML support. That’s not very helpful if you have a legitimate use for XML.

In 2004 and 2005, Ecma published ECMA-357 which was a spec for handling XML in Javascript. The spec was called E4X (Ecmascript for XML). This was adopted by Firefox and ActionScript. Unfortunately it was not adopted by Chrome, so it never became a Javascript standard. Royale supports the full E4X spec and XML is treated as a first class citizen in Royale. This makes dealing with XML much easier. Read the page on XML for full details.

Royale also has a lightweight JXON class which can be useful for simple reading of XML if you don’t need all the functionality of E4X.


Type casting

Basics about type casting…

is and as

Advantages and considerations…

Kitchen sink

To include or not?

what else….