This is a Webpack loader for Flowcheck .

Flowcheck is a runtime assertions library that uses Flow syntax but is meant to be ran in development.

npm install --save-dev flowcheck-loader

Chain this loader with jsx?stripTypes so types are stripped in JS code:

// Webpack config
module.exports = {
 ...
 module: {
 loaders: [
 { test: /\.jsx?$/, loaders: ['jsx?harmony&stripTypes', 'flowcheck'], exclude: /node_modules/ }
 ]
 }
};

For production builds, just leave flowcheck out so jsx?stripTypes strips them completely.

You can use this with or without using Flow or TypeScript itself.
It can also be a nice first step towards migrating code to Flow.

Read more about Flowcheck