reconciler.js

// ...code
function beginWork(wipFiber) {
  if (wipFiber.tag == CLASS_COMPONENT) {
    updateClassFiber(wipFiber);
  } else {
    updateHostFiber(wipFiber);
  }
}

function updateHostFiber(wipFiber) {
  if (!wipFiber.stateNode) {
    // if this is the initialRender and stateNode is null
    // create a new node.
    wipFiber.stateNode = createDomElement(wipFiber);
  }
  const newChildElements = wipFiber.props.children;
  reconcileChildrenArray(wipFiber, newChildElements);
}

function updateClassFiber(wipFiber) {
  let instance = wipFiber.stateNode;
  if (instance == null) {
    // if this is the initialRender call the constructor
    instance = wipFiber.stateNode = createInstance(wipFiber);
  } else if (wipFiber.props == instance.props && !wipFiber.partialState) {
    // nothing has changed here
    // lets move to the children
    cloneChildFibers(wipFiber);
    return;
  }

  instance.props = wipFiber.props;
  instance.state = Object.assign({}, instance.state, wipFiber.partialState);
  wipFiber.partialState = null;

  const newChildElements = wipFiber.stateNode.render();
  reconcileChildrenArray(wipFiber, newChildElements);
}

function createInstance(fiber) {
  //similar to the previous implementation
  // we instanciate a new object of the class provided in the
  // type prop and return the new instance
  const instance = new fiber.type(fiber.props);
  instance.__fiber = fiber;
  return instance;
}

function createDomElement(fiber) {
  // check the type of the fiber object.
  const isTextElement = fiber.type === TEXT_ELEMENT;
  const dom = isTextElement
    ? document.createTextNode("")
    : document.createElement(fiber.type);
  updateDomProperties(dom, [], fiber.props);
  return dom;
}