Type-checking can be a useful asset in a Javascript project. A type checker can catch problems that are introduced when adding features or refactoring, which can reduce the amount of time spent debugging and testing. Type annotations provide a form of always-up-to-date documentation that makes it easier for developers to understand an unfamiliar code base. But it is important to use type-checking effectively to get its full benefit. The Javascript community is fortunate to have a choice of two great type checkers. These recipes focus on Flow, and introduce patterns for using Flow effectively. …

Flow has some very interesting features that are currently not documented. It is likely that the reason for missing documentation is that these features are still experimental. Caveat emptor. I took a stroll through the source code for Flow v0.11. Here is what I found while reading type_inference_js.ml and react.js. …