Anders published a video of his new project today, which aims at creating a superset of JavaScript, that compiles down to regular current JavaScript. Anders is a tremendously clever guy, and it always shows in his work. There is much to like in the enterprise (good code completion, refactoring and adoption of the module pattern instead of namespaces to name three), but a few things made me rise an eyebrow.

First, there is no mention of CoffeeScript or Dart, but he does talk briefly about Script# and GWT. This is probably because the target audience seems to be the same as the audience for the latter two, i.e. developers who are more comfortable with statically-typed languages such as C# and Java than dynamic languages such as JavaScript. I don’t think he’s aiming at JavaScript developers. Classes and interfaces, although well executed, are not especially appealing.

Second, as any code generation tool (and this is true of CoffeeScript as well), you’d better like the generated code. I didn’t, unfortunately. The code that I saw is not the code I would have written. What’s more, I didn’t always find the TypeScript code especially more expressive than what it gets compiled to.

I also have a few questions.

Is it possible to duck-type interfaces? For example, if I have an IPoint2D interface with x and y coordinates, can I pass any object that has x and y into a function that expects IPoint2D or do I need to necessarily create a class that implements that interface, and new up an instance that explicitly declares its contract? The appeal of dynamic languages is the ability to make objects as you go. This needs to be kept intact.

More technical: why are generated variables and functions prefixed with _ rather than the $ that the EcmaScript spec recommends for machine-generated variables?

In conclusion, while this is a good contribution to the set of ideas around JavaScript evolution, I don’t expect a lot of adoption outside of the devoted Microsoft developers, but maybe some influence on the language itself. But I’m often wrong. I would certainly not use it because I disagree with the central motivation for doing this: Anders explicitly says he built this because “writing application-scale JavaScript is hard”. I would restate that “writing application-scale JavaScript is hard for people who are used to statically-typed languages”. The community has built a set of good practices over the last few years that do scale quite well, and many people are successfully developing and maintaining impressive applications directly in JavaScript.

You can play with TypeScript here: http://www.typescriptlang.org