The article talks about TypeScript vs. Flow as well - TS has a lot more popularity than Flow currently, and it only looks like the gap is going to grow larger.

The article talks about TypeScript vs. Flow as well - TS has a lot more popularity than Flow currently, and it only looks like the gap is going to grow larger. Types also allows for auto-generation of docs that are guaranteed to reflect the current state of the code. For a library maintainer, a maintainer can just release an ES5 version of the library when publishing via npm - the user then gets the benefit of choosing between TS or ES5. It can also simultaneously be released as ES6 if desired. I should mention that it doesn't make sense for pre-existing libraries to migrate to TS (or anything) if it is already in JS and is a largely used library without a specific end goal that is to be achieved - the article is wrong about that, open source doesn't have infinite resources unfortunately.