Inspiration was heavily taken from react-elm-components. This package takes advantage of React Hooks to provide a lightweight (282 bytes) solution.

After you have compiled an Elm program to JavaScript, you can embed it in React like this:

import ElmComponent from 'react-elm-component'
import { Elm } from './ElmApp.elm'

function render() {
	return <ElmComponent app={Elm.Todo} />
}

Sometimes you want to give your Elm program some flags on start up. For example, maybe your Todo module needs to get an array of todos. You would write something like this:

import ElmComponent from 'react-elm-component'
import { Elm } from './ElmApp.elm'

function render() {
	var flags = { todos: ["Get Milk", "Do Laundry"] };
	return <ElmComponent app={Elm.Todo} flags={flags} />
}

These flags will be given to the Elm program, allowing you to do some setup work in JS first.

As your Elm program gets fancier, you will probably need to interact with JavaScript. We do this with ports. Think of these as holes in the side of an Elm program that let you pass messages back-and-forth.

So maybe we extend our Todo app to allow outsiders to register new tasks through the todos port. And maybe we also expose numActiveTodos so that the outsider can know how much work you have left. You would set it up like this:

import ElmComponent from 'react-elm-component'
import { Elm } from './ElmApp.elm'

function render() {
	return <ElmComponent app={Elm.Todo} ports={setupPorts} />
}

function setupPorts(ports) {
	ports.numActiveTodos.subscribe(function(n) {
		console.log(n);
	});

	ports.todos.send("Invent the Universe");
	ports.todos.send("Bake an Apple Pie");
}

In the setupPorts function, we first subscribe to the numActiveTodos port. Whenever the number of active todos changes, we will run that function and log the number on the console. After that, we send two values through the todos port. This will add both of these into the model and trigger the numActiveTodos callback twice.