code: function mountClassInstance(workInProgress, priorityLevel) {
      var current = workInProgress.alternate;
  
      {
        checkClassInstance(workInProgress);
      }
  
      var instance = workInProgress.stateNode;
      var state = instance.state || null;
  
      var props = workInProgress.pendingProps;
      !props ? invariant(false, 'There must be pending props for an initial mount. This error is likely caused by a bug in React. Please file an issue.') : void 0;
  
      var unmaskedContext = getUnmaskedContext$2(workInProgress);
  
      instance.props = props;
      instance.state = state;
      instance.refs = emptyObject;
      instance.context = getMaskedContext$2(workInProgress, unmaskedContext);
  
      if (ReactFeatureFlags_1.enableAsyncSubtreeAPI && workInProgress.type != null && workInProgress.type.prototype != null && workInProgress.type.prototype.unstable_isAsyncReactComponent === true) {
        workInProgress.internalContextTag |= AsyncUpdates$1;
      }
  
      if (typeof instance.componentWillMount === 'function') {
        callComponentWillMount(workInProgress, instance);
        // If we had additional state updates during this life-cycle, let's
        // process them now.
        var updateQueue = workInProgress.updateQueue;
        if (updateQueue !== null) {
          instance.state = beginUpdateQueue$2(current, workInProgress, updateQueue, instance, state, props, priorityLevel);
        }
      }
      if (typeof instance.componentDidMount === 'function') {
        workInProgress.effectTag |= Update$1;
      }
    }
docs: Invokes the mount life-cycles on a previously never rendered instance.