reconciler.js

const HOST_ROOT = "root";
const HOST_COMPONENT = "host";

// code..

export function render(elements, containerDom) {
  workQueue.push({
    from: HOST_ROOT, // the root/parent fiber
    dom: containerDom, // document.getElementById("app") just a dom node where this fiber will be appended to as a child
    newProps: { children: elements }
  });
  requestIdleCallback(performWork);
}