Flow is a static typechecker for JavaScript.

To find out more about Flow, check out https://flow.org.

Flow adds static typing to JavaScript to improve developer productivity and code quality. In particular, static typing offers benefits like early error checking, which helps you avoid certain kinds of runtime failures, and code intelligence, which aids code maintenance, navigation, transformation, and optimization.

We have designed Flow so developers can reap its benefits without losing the "feel" of coding in JavaScript. Flow adds minimal compile-time overhead, as it does all its work proactively in the background. And Flow does not force you to change how you code — it performs sophisticated program analysis to work with the idioms you already know and love.