I wrote an article last year, Consider Using TypeScript, to explain some of the benefits of using TypeScript and I'm just curious for those in the JS ecosystem if you use a language like TypeScript, Reason ML, or Facebook's Flow type checker.
This short post on the 2ality blog is relevant and a good read as well.
If you currently don't use one of the above, are you considering trying one of them? If you did try or use one of them, what kinds of issues, if any did you have? Difficult to setup? Learning curve etc.