Resolve promise when document onload

awaitDOMContentLoaded.js

// Resolve promise when document onload
function awaitDOMContentLoaded() {
  return new Promise(function (resolve) {
    if (document.readyState === "complete") {
      resolve();
    } else {
      document.addEventListener("DOMContentLoaded", resolve);
    }
  });
}

awaitDOMContentLoaded()
  .then(process1)
  .then(process2)
  .then(...)