The scoping with modules and the typing really helped since I'm familiar with C# and having "everything light up" in the IDE. Saved a ton of time letting a compiler catch things instead of writing tests.
I tried coffee script, but found the copy/paste option for javascript that I have already written won me over.
And when valid javascript is valid typescript, it seems odd to not opt to use it when the only addition is a compile step to ensure sanity.
No longer am I concerned about errant semi-colons or a stupid typo in a method name on an error handling path that a unit test missed.
We found a bunch of real and potential bugs due to JavaScript's dynamic nature, particularly with regards to the number of function arguments (JavaScript being happy to ignore extra parameters or replace missing parameters with undefined).
Saved a ton of time letting a compiler catch things instead of writing tests.
Type checking, or static analysis in general, checks for the presence of a particular class of errors.
Unit tests check for correct behaviour, and in doing so imply a lack of any errors, types or otherwise.
While it can be good to use unit tests to check for regressions in a specific error, the core of your tests should be specifying the behaviour of your system, not trying to catch out any specific class of errors.
If your tests are busy looking for specific errors, there will always be some that you miss (type checking, even in Haskell, misses a lot of classes of error). If your tests check for correct behaviour, there can be no errors.
Of course, writing tests with perfect coverage and specifying all the behaviour is impossible, even more so when you've got actual features to get out the door. So any static analysis you're comfortable with can be great for making up some of the slack, but let's not pretend that in most cases we shouldn't now need to be writing those tests to check it does what it's meant to.
The place where this approach sometimes falls short is on the public API boundary, as I mentioned in my other post. Types can be useful there, so maybe my original assertion was a little too strong, but still types should very rarely mean less tests.
(Apologies if the post is a bit all over the place, redrafting on a phone is hard)
type annotation tests that at compile time
There might be some worth in checking the structure of an object (including attached functions) as it leaves a public api, especially in a language like Javascript that plays it so fast and loose with object construction.
That's one of the nice thing about typescript - you can use it where it's most helpful and leave it where it's not.
especially for testing return types as C for example you assign those.
In C it's more popular to write example code which uses the library, and the errors are spotted at compile time.
I developed the first prototype in pure JS and there were some bugs (due to me being new to this FireBase model, and some hastily developed code).
I then converted the whole thing to TypeScript, because I prefer static typing. It took some time to setup: upgrading to the latest nodejs, collecting all type definitions for popular libraries, etc. The DefinitelyTyped[1] library is a great resource. But there were some warts in it. I had to make some PRs to this library for some missing methods in jQuery.
But right after finishing the setup, I was able to find and fix a complicated sequencing bug. That alone was worth the trouble of the setup cost.
I then tried using TypeScript for another project (this one had a licensed IDEA available). Here the setup cost was even lower. There are plugins available for Typescript in IDEA. Though I couldn't convince everyone in the team to switch to TypeScript.
This was back before they added generics support. Now that generics is added, I think there is hardly any excuse left to not use TypeScript (or a similar alternative).
What would make TypeScript awesome is support from more IDEs (Eclipse especially).
There exists a plugin for eclipse, but I have never tried it. I'm mostly an Intellij IDEA user.
The compiler speed is OK (well, 0.9.0 wasn't, but 0.9.1 should be back to where it was before.) The main pain point is that we usually want to type any 3rd party JS we interface with (e.g. Angular, Backbone, Highcharts etc.) though https://github.com/borisyankov/DefinitelyTyped is helpful with this. No major complaints as of yet.
It's still perhaps too early to tell how long MS will be supporting it. Windows 8 has a JS interface which is comforting, but they could turn around and decide to stop development on it. That would be a great shame, but the code is open source, the compiler works well as of today, and the generated JS is readable enough that it can be edited.
We're slowly migrating over from Coffeescript.
http://blogs.msdn.com/b/bharry/archive/2012/10/24/typescript...
A real aid to productivity in my opinion. And I love how you can turn up and down the typing as you see fit.
The benefits of inferred typing is great. Mostly you just need to explicitly declare the types of your class properties, and then code that uses the properties or modifies them (like .map() or .forEach) doesn't need explicit types defined - so it still looks like JavaScript.
Refactoring is no longer a search and replace in files (and pray tests pass) affair.
Dealing with other programmer's code is no longer as mentally taxing.
The optional typing ability is a life saver when your code base grows. We've used the integrated tooling with Visual Studio which also helped a lot.
Microsoft is promising it will be kept in sync with ECMAScript 6, so we don't really think there is much risk in using it. It's definitely helped with productivity, and the learning curve isn't high for JavaScript developers.