A simple root-application.js file for single-spa

single-spa-root-application.js

import {declareChildApplication, start} from 'single-spa';

// Register your first application with single-spa. More apps will be registered as you create them
declareChildApplication("cool-app", loadCoolApp, isCoolAppActive);

// Tell single-spa that you're ready for it to mount your application to the DOM
start();

// This is a "loading function"
function loadCoolApp() {
  return import("./cool-app/cool.app.js");
}

// This is an "activity function"
function isCoolAppActive() {
  return window.location.hash.startsWith("#/cool");
}

Your medium article that references this is almost two years old, but it still stands as one of the best annotated guides. Consider updating declareChildApplication to registerApplication.