Function createFiber

docs/external/js/react-dom.development.js:23797–23800

(tag, pendingProps, key, mode)

  var createFiber = function (tag, pendingProps, key, mode) {
    // $FlowFixMe: the shapes are exact here but Flow doesn't like constructors
    return new FiberNode(tag, pendingProps, key, mode);
  };

  function shouldConstruct(Component) {
    var prototype = Component.prototype;