reconciler.js

// ...code

let pendingCommit = null; // this is what will be flushed to the dom
// ... code


function completeWork(fiber) {
  // this function takes the list of effects of the children and appends them to the effects of
  // the parent
  if (fiber.tag == CLASS_COMPONENT) {
    // update the stateNode.__fiber of the 
    // class component to the new wipFiber (it doesn't deserve this name anymore since we are done with the work we needed to do to it)
    fiber.stateNode.__fiber = fiber;
  }

  if (fiber.parent) {
    // append the fiber's child effects to the parent of the fiber
    // the effects of the childFiber
    // are appended to the fiber.effects
    const childEffects = fiber.effects || [];
    // if the effectTag is not present of this fiber, if there are none,
    // then return an empty list
    const thisEffect = fiber.effectTag != null ? [fiber] : [];
    const parentEffects = fiber.parent.effects || [];
    // the new parent effects consists of this current fiber's effects +
    // effects of this current Fiber + the parent's own effects
    fiber.parent.effects = parentEffects.concat(childEffects, thisEffect);
  } else {
    // if the fiber does not have a parent then it means we
    // are at the root. and ready to flush the changes to the dom.
    pendingCommit = fiber;
  }
}