You could start incrementally with Tx3's suggestion - > One thing I would like to add regarding the .js files, TypeScript 1.8 added a flag --allowJs which make

You could start incrementally with Tx3's suggestion - > One thing I would like to add regarding the .js files, TypeScript 1.8 added a flag --allowJs which makes some sanity checks to the plain old .js files. Or burai's suggestion - > Another good alternative is Facebook's flow. Rather than being a transpired language it works on top of JavaScript files.