I haven't used Flow (we actually couldn't get it to work with our build system in a few days and gave up, because TypeScript was easy). We integrated replaced

I haven't used Flow (we actually couldn't get it to work with our build system in a few days and gave up, because TypeScript was easy). We integrated replaced Babel with TypeScript in about one day, and ported the entire codebase from ES6 to TS in about two or three days. The code looks almost identical except for the type annotations, which we were documenting in comments anyway. Also, you can pretty much switch right back to Babel if you want, since Babel will ignore type annotations. Editor support for TS is amazing. Both Atom and VS Code have great autocompletion and live typechecking as you edit. Build times with TS are okay. Our codebase rebuilds with ts-loader in webpack within something like 4 seconds upon saving. I'd prefer half a second of course. We have had no problem using any existing JS code. You can provide type annotations or not, but you don't have to. We use React, moment.js, underscore, jQuery. All the major libraries have type bindings. We also use some internal CoffeeScript libraries and occasionally bother providing type bindings, but usually not. In short, TypeScript is great and if you're going to adopt ES6 you might as well adopt TypeScript too.