code: function(internalInstance, rootID, transaction, context) {
      var markup = internalInstance.mountComponent(rootID, transaction, context);
      if ("production" !== process.env.NODE_ENV) {
        ReactElementValidator.checkAndWarnForMutatedProps(
          internalInstance._currentElement
        );
      }
      transaction.getReactMountReady().enqueue(attachRefs, internalInstance);
      return markup;
    }
docs: Initializes the component, renders markup, and registers event listeners.
  
  @param {ReactComponent} internalInstance
  @param {string} rootID DOM ID of the root node.
  @param {ReactReconcileTransaction|ReactServerRenderingTransaction} transaction
  @return {?string} Rendered markup to be inserted into the DOM.
  @final
  @internal