Calypso could migrate to TypeScript to help with maintainability and learnability of the codebase.
=== More details
There are people who might strongly disagree with the value of TypeScript, as in this essay "TypeScript isn't the answer", so the issue is not without controversy. Comments there tend to support TypeScript though. As David Kapetyan says (and reflects my own experience with TypeScript):
Ideally, the initial conversion process should not take that long (mostly renaming file extensions, sometimes changing imports, adding some basic stub typing for libraries, setting TypeScript compiler options, configuring your IDE, and adding another step of calling the TypeScript compiler to the build process). That whole process for Calypso should probably not take much more than a week or so if a few people did it together who were already familiar with the Calypso codebase. Then further improvements to Calypso could be done incrementally. Helping with that might make a good Automattic trial project for someone. :-)
There are downsides to working in TypeScript in that debugging can become a bit harder -- even with source maps (which also slow the build process if you generate them). However, overall, that tradeoff of slightly harder debugging for much easier refactoring and also catching many more errors before they make it to runtime and even production seems worth it to many people.
TypeScript 1.6 would be required to get JSX support for the current Calypso codebase. This issue is a spinoff from issue #650, although migrating to TypeScript would make any sort of major refactoring of Calypso easier. Moving Calypso to TypeScript is just a good thing to do for multiple reasons.
Thanks for the thought, but that's not on our radar at this time.
@apeatling Thanks for the reply. As something to think about down the road, what sort of metrics or other evidence about developer happiness could put something like TypeScript or something similar on the radar?
Maybe trending twitters? :-) [Although this one is from someone at Microsoft...]
"Happiness is coding in Typescript all day long."
Likewise, how could metrics about code quality, user experience bug reports, or time to do refactoring be linked to a consideration of TypeScript (or whatever)? I'm not asking that to get an answer now. I'm just suggesting it as a question to think about in the future, given Automattic (sensibly) likes using metrics for decision making when possible (even given, as Scott Berkun pointed out, there is no easy-to-use metric for deciding what metrics to use or how to weigh competing metrics).
No doubt Automattic will continue to reflect on emerging technologies and their various benefits and risks as time goes by. So, I hope you'll revisit the decision in a year or so, or perhaps before then even try TypeScript in some small part of the Calypso codebase or on another project and see how it goes.
In any case, Calypso is a great move forward for WordPress, whatever it is coded in. :-) Thanks for providing the leadership to make that happen.