performunitofwork

reconciler.js

// ... code
function performUnitOfWork(wipFiber) {
  // lets work on the fiber
  beginWork(wipFiber);
  if (wipFiber.child) {
    // if a child exists its passed on as
    // the nextUnitOfWork
    return wipFiber.child;
  }

  // No child, we call completeWork until we find a sibling
  let uow = wipFiber;
  while (uow) {
    completeWork(uow); // completework on the currentFiber
    // return the siblings of the currentFiber to
    // be the nextUnitOfWork
    if (uow.sibling) {
      // Sibling needs to beginWork
      return uow.sibling;
    }
    // if no siblings are present,
    // lets climb up the tree as we call completeWork
    // when no parent is found / if we've reached the top,
    // this function returns null and thats how we know that we have completed
    // working on the work in progress tree.
    uow = uow.parent;
  }
}