function beginWork(
  current: Fiber | null,
  workInProgress: Fiber,
  renderExpirationTime: ExpirationTime,
): Fiber | null {
  const updateExpirationTime = workInProgress.expirationTime

  if (current !== null) {
    const oldProps = current.memoizedProps
    const newProps = workInProgress.pendingProps
    if (
      oldProps === newProps &&
      !hasLegacyContextChanged() &&
      (updateExpirationTime === NoWork ||
        updateExpirationTime > renderExpirationTime)
    ) {
      // This fiber does not have any pending work. Bailout without entering
      // the begin phase. There's still some bookkeeping we that needs to be done
      // in this optimized path, mostly pushing stuff onto the stack.
      switch (workInProgress.tag) {
        case HostRoot:
          pushHostRootContext(workInProgress)
          resetHydrationState()
          break
        case HostComponent:
          pushHostContext(workInProgress)
          break
        case ClassComponent: {
          const Component = workInProgress.type
          if (isLegacyContextProvider(Component)) {
            pushLegacyContextProvider(workInProgress)
          }
          break
        }
        case HostPortal:
          pushHostContainer(
            workInProgress,
            workInProgress.stateNode.containerInfo,
          )
          break
        case ContextProvider: {
          const newValue = workInProgress.memoizedProps.value
          pushProvider(workInProgress, newValue)
          break
        }
        case Profiler:
          if (enableProfilerTimer) {
            workInProgress.effectTag |= Update
          }
          break
        case SuspenseComponent: {
          const state: SuspenseState | null = workInProgress.memoizedState
          const didTimeout = state !== null && state.didTimeout
          if (didTimeout) {
            const primaryChildFragment: Fiber = (workInProgress.child: any)
            const primaryChildExpirationTime =
              primaryChildFragment.childExpirationTime
            if (
              primaryChildExpirationTime !== NoWork &&
              primaryChildExpirationTime <= renderExpirationTime
            ) {
              return updateSuspenseComponent(
                current,
                workInProgress,
                renderExpirationTime,
              )
            } else {
              const child = bailoutOnAlreadyFinishedWork(
                current,
                workInProgress,
                renderExpirationTime,
              )
              if (child !== null) {
                return child.sibling
              } else {
                return null
              }
            }
          }
          break
        }
      }
      return bailoutOnAlreadyFinishedWork(
        current,
        workInProgress,
        renderExpirationTime,
      )
    }
  }

  // Before entering the begin phase, clear the expiration time.
  workInProgress.expirationTime = NoWork

  switch (workInProgress.tag) {
    case IndeterminateComponent: {
      const elementType = workInProgress.elementType
      return mountIndeterminateComponent(
        current,
        workInProgress,
        elementType,
        renderExpirationTime,
      )
    }
    case LazyComponent: {
      const elementType = workInProgress.elementType
      return mountLazyComponent(
        current,
        workInProgress,
        elementType,
        updateExpirationTime,
        renderExpirationTime,
      )
    }
    case FunctionComponent: {
      const Component = workInProgress.type
      const unresolvedProps = workInProgress.pendingProps
      const resolvedProps =
        workInProgress.elementType === Component
          ? unresolvedProps
          : resolveDefaultProps(Component, unresolvedProps)
      return updateFunctionComponent(
        current,
        workInProgress,
        Component,
        resolvedProps,
        renderExpirationTime,
      )
    }
    case ClassComponent: {
      const Component = workInProgress.type
      const unresolvedProps = workInProgress.pendingProps
      const resolvedProps =
        workInProgress.elementType === Component
          ? unresolvedProps
          : resolveDefaultProps(Component, unresolvedProps)
      return updateClassComponent(
        current,
        workInProgress,
        Component,
        resolvedProps,
        renderExpirationTime,
      )
    }
    case HostRoot:
      return updateHostRoot(current, workInProgress, renderExpirationTime)
    case HostComponent:
      return updateHostComponent(current, workInProgress, renderExpirationTime)
    case HostText:
      return updateHostText(current, workInProgress)
    case SuspenseComponent:
      return updateSuspenseComponent(
        current,
        workInProgress,
        renderExpirationTime,
      )
    case HostPortal:
      return updatePortalComponent(
        current,
        workInProgress,
        renderExpirationTime,
      )
    case ForwardRef: {
      const type = workInProgress.type
      const unresolvedProps = workInProgress.pendingProps
      const resolvedProps =
        workInProgress.elementType === type
          ? unresolvedProps
          : resolveDefaultProps(type, unresolvedProps)
      return updateForwardRef(
        current,
        workInProgress,
        type,
        resolvedProps,
        renderExpirationTime,
      )
    }
    case Fragment:
      return updateFragment(current, workInProgress, renderExpirationTime)
    case Mode:
      return updateMode(current, workInProgress, renderExpirationTime)
    case Profiler:
      return updateProfiler(current, workInProgress, renderExpirationTime)
    case ContextProvider:
      return updateContextProvider(
        current,
        workInProgress,
        renderExpirationTime,
      )
    case ContextConsumer:
      return updateContextConsumer(
        current,
        workInProgress,
        renderExpirationTime,
      )
    case MemoComponent: {
      const type = workInProgress.type
      const unresolvedProps = workInProgress.pendingProps
      const resolvedProps = resolveDefaultProps(type.type, unresolvedProps)
      return updateMemoComponent(
        current,
        workInProgress,
        type,
        resolvedProps,
        updateExpirationTime,
        renderExpirationTime,
      )
    }
    case SimpleMemoComponent: {
      return updateSimpleMemoComponent(
        current,
        workInProgress,
        workInProgress.type,
        workInProgress.pendingProps,
        updateExpirationTime,
        renderExpirationTime,
      )
    }
    case IncompleteClassComponent: {
      const Component = workInProgress.type
      const unresolvedProps = workInProgress.pendingProps
      const resolvedProps =
        workInProgress.elementType === Component
          ? unresolvedProps
          : resolveDefaultProps(Component, unresolvedProps)
      return mountIncompleteClassComponent(
        current,
        workInProgress,
        Component,
        resolvedProps,
        renderExpirationTime,
      )
    }
    default:
      invariant(
        false,
        'Unknown unit of work tag. This error is likely caused by a bug in ' +
          'React. Please file an issue.',
      )
  }
}