You probably don't need or shouldn't use this. If you do, you need to be experienced in JS and Elm, and thread carefully to not break purity.
This works for 0.19, and it is likely to break or not even work in future versions of Elm. If you use this you are solely responsible for any future pain you may incur on by not being able to easily migrate your app to new Elm versions.
See src/Main.elm for examples of usage of the sync and async FFI. Within the
boundaries of possibility, the library still tries to encapsulate JS code
running to not break the Elm runtime or your app.
Live example from the Main.elm code
If you want to add this to your application, copy src/Js.elm to your app, and
include the JS file in public/elm-js-interop.js.
Stay safe out there.