Function createFiber

code/controlled-uncontrolled/public/app.js:6215–6218

(tag, pendingProps, key, mode)

// 5) It should be easy to port this to a C struct and keep a C implementation
//    compatible.
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) {
  return !!(Component.prototype && Component.prototype.isReactComponent);