ReactFiberCount.js

function countReactFibers() {
  const rootContainer = document.getElementById('root');
  const root = rootContainer._reactRootContainer;
  const internalRoot = root._internalRoot;

  let fiberCount = 0;
  let elementCount = 0;

  function travelFibers(fiberNode) {
    console.log(fiberNode);
    if (typeof fiberNode.elementType === 'string') {
      elementCount += 1;
    }

    if (typeof fiberNode.elementType === 'function') {
      fiberCount += 1;
    }

    if (fiberNode.child) {
      travelFibers(fiberNode.child);
    }

    if (fiberNode.sibling) {
      travelFibers(fiberNode.sibling);
    }
  }

  travelFibers(internalRoot.current);

  console.info('Fiber:', fiberCount);
  console.info('ElementCount:', elementCount);
}