Typesafe Javascript

You should take a look at the haxe project.

Haxe is a very nice typed language that uses type inference (i.e. you're not forced to write a lot of type declarations) but that enforces type correctness at compile time.

The language has a javascript-like syntax and the compiler can generate code for the neko virtual machine, for javascript, as3, c++ or PHP.

Update

Today the most popular choice is probably Typescript, a superset of Javascript that allows optional type declarations that are enforced compile time.

GWT does what looking for, but its a way oversized for the most cases. You could take a look at googles closure framework which fakes the typed safe with anotations

Whilst I'm a little late to this party, I think it's definitely worth mentioning Dart (which is a Google product) and TypeScript (which is a Microsoft product).

JavaScript is fast becoming an extremely popular language as applications become more web based. However as you have pointed out, JavaScript lacks type safety, and to name a few other things; classes, interfaces and packages/namespaces/modules.

This is where Dart and TypeScript step in. These languages are essentially supersets of JavaScript. When you write Dart or TypeScript code, it is compiled into clean, standards compliant JavaScript.

The benefits of Dart and TypeScript are that they provide type safety, interfaces, classes etc. Thus allowing you to write cleaner, scalable, manageable applications, which still run in the browser.

Being a Microsoft oriented programmer, I've had a lot of experience with TypeScript, including being somewhat active in the development of the language (you can find information for TypeScript development at codeplex)

My only concern at the moment is that TypeScript is lacking in some fundamental features. It seems that some of the current implementation (0.9.0 alpha) has some equally gaping holes that might deter the savvy developer from using it at the moment (subject to change of course).

I cannot really comment on Dart, as I have only used this a few times, but my overall experience with Dart was good!