Elm has JS interop via ports, which would allow you to mix or migrate towards elm.

https://evancz.gitbooks.io/an-introduction-to-elm/content/in...

Emphasis on the migrate, you do not want to have app with both JS and Elm for a long term perhaps.

In the main article: "I do not expect to be compiling to JavaScript forever, especially with WebAssembly on the horizon. The smaller the interface between Elm and JS, the easier it will be to support other platforms."

While I don't want to sound doom and gloom, the JavaScript support according to article is anomaly of today's browsers. So if you want to rely on two parts JS and one parts Elm type of thing, it's good to know it maybe gone.

I would take a look at koajs. It's a simpler, smaller framework with a killer feature: JavaScript Generators. Which means you don't have to use callbacks.

http://koajs.com/