useOptimistic: How to handle race conditions when multiple optimistic updates affect the same state slice?

Question

I'm building a collaborative task management app using React 19's useOptimistic hook. Multiple users can update task properties (status, priority, assignee) simultaneously. I'm running into race conditions where optimistic updates overwrite each other, leading to inconsistent UI state.

Environment React 19.0.0 Next.js 15.1.0 Server Actions for mutations

When two rapid updates target the same task but different properties, the second optimistic update replaces the first one entirely instead of merging changes. This causes the UI to briefly show incorrect data until the server response arrives.

here is the code:

'use client'; import { useOptimistic } from 'react'; import { updateTaskStatus, updateTaskPriority } from './actions'; function TaskItem({ task }) { const [optimisticTask, setOptimisticTask] = useOptimistic( task, (state, newTaskData) => ({ ...state, ...newTaskData }) ); const handleStatusChange = async (newStatus) => { setOptimisticTask({ status: newStatus }); await updateTaskStatus(task.id, newStatus); }; const handlePriorityChange = async (newPriority) => { setOptimisticTask({ priority: newPriority }); await updateTaskPriority(task.id, newPriority); }; return ( <div> <h3>{optimisticTask.title}</h3> <select value={optimisticTask.status} onChange={(e) => handleStatusChange(e.target.value)} > <option value="todo">To Do</option> <option value="in-progress">In Progress</option> <option value="done">Done</option> </select> <select value={optimisticTask.priority} onChange={(e) => handlePriorityChange(e.target.value)} > <option value="low">Low</option> <option value="medium">Medium</option> <option value="high">High</option> </select> <p>Status: {optimisticTask.status}</p> <p>Priority: {optimisticTask.priority}</p> </div> ); }

Problem: The UI briefly shows status: "todo" (original) because the second optimistic update didn't include status

Question:

What is the recommended pattern for handling multiple concurrent optimistic updates in React 19's use Optimistic hook when updates can arrive out of order and affect overlapping state? Should I implement a pending updates queue with merge logic, or does React 19 provide built-in reconciliation mechanisms I'm not aware of?

Drew Reese · Accepted Answer · 2025-10-29 01:07:38Z

It appears you are missing the startTransition or useTransition hook^* API to actually handle the asynchronous optimistic transition. Adding a transition should help manage merging the "in-flight" optimistic updates for you. You enqueue the optimistic state update and wrap the asynchronous logic in a transition.

Examples:

import { useOptimistic, startTransition } from 'react'; function TaskItem({ task }) { const [optimisticTask, setOptimisticTask] = useOptimistic( task, (state, newTaskData) => ({ ...state, ...newTaskData }) ); const handleStatusChange = (newStatus) => { setOptimisticTask({ status: newStatus }); startTransition(async () => { await updateTaskStatus(task.id, newStatus); }); }; const handlePriorityChange = (newPriority) => { setOptimisticTask({ priority: newPriority }); startTransition(async () => { await updateTaskPriority(task.id, newPriority); }); }; ... }

import { useOptimistic, useTransition } from 'react'; function TaskItem({ task }) { const [isPending, startTransition] = useTransition(); const [optimisticTask, setOptimisticTask] = useOptimistic( task, (state, newTaskData) => ({ ...state, ...newTaskData }) ); const handleStatusChange = (newStatus) => { setOptimisticTask({ status: newStatus }); startTransition(async () => { await updateTaskStatus(task.id, newStatus); }); }; const handlePriorityChange = (newPriority) => { setOptimisticTask({ priority: newPriority }); startTransition(async () => { await updateTaskPriority(task.id, newPriority); }); }; ... {isPending && <div>Updating Task</div<} }

_{^*useTransition also provides an isPending flag that tells you whether there is a pending Transition, otherwise it returns the same startTransition function that lets you mark updates as a transition.}

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