Sofortiges Feedback mit useOptimistic
Wenden Sie optimistische UI-Aktualisierungen für Likes, Kommentare und Umschalter an, während die Action im Hintergrund abgeschlossen wird.
Sofortiges Feedback mit useOptimistic ist eine kostenlose Next.js 15 Fullstack (App Router + Server Actions)-Lektion auf CoddyKit. Dies ist Lektion 4 von 4. Du kannst die komplette Lektion unten kostenlos lesen – dann übst du sie direkt im Browser mit einem integrierten Code-Editor und einem KI-Tutor rund um die Uhr. Sie ist Teil des Next.js 15 Fullstack (App Router + Server Actions)-Lernpfads, und dein Fortschritt wird über Web und CoddyKit-App synchronisiert. Der Next.js 15 Fullstack (App Router + Server Actions)-Kurs umfasst insgesamt 4 Lektionen.
Teile dieser Lektion wurden noch nicht übersetzt und werden auf Englisch angezeigt.
The Latency Problem
When a user clicks a like button, the change has to travel to the server and back before the UI reflects it. On a slow network that delay feels broken.
The fix is optimistic UI: assume the action will succeed, update the screen instantly, and reconcile with the server once it responds. If the server fails, you roll back automatically.
React 19 (shipped with Next.js 15) gives you a dedicated hook for this: useOptimistic.
What useOptimistic Returns
useOptimistic takes your real state and an update function, and returns a temporary optimistic value plus a function to apply optimistic changes.
optimisticState— what you render right nowaddOptimistic(value)— queue an optimistic change
The signature is useOptimistic(state, (currentState, optimisticValue) => newState). The reducer-style function merges the optimistic value into the current state.
const [optimisticState, addOptimistic] = useOptimistic(
state,
(currentState, optimisticValue) => {
// return the new state to display optimistically
return { ...currentState, ...optimisticValue };
}
);A Like Button: The Server Action
First, the real work happens in a Server Action. It runs on the server, mutates the database, and revalidates the cache.
Notice the "use server" directive and the revalidatePath call so other parts of the app see the fresh count.
"use server";
import { revalidatePath } from "next/cache";
import { db } from "@/lib/db";
export async function toggleLike(postId: string, liked: boolean) {
await db.post.update({
where: { id: postId },
data: { likes: { increment: liked ? 1 : -1 } },
});
revalidatePath("/feed");
}Wiring useOptimistic to the Button
In a Client Component, we hold the optimistic like state. When clicked, we call addOptimistic first (instant UI), then await the Server Action.
If the action throws, React automatically discards the optimistic value and reverts to the real likes prop.
"use client";
import { useOptimistic, useTransition } from "react";
import { toggleLike } from "./actions";
export function LikeButton({ postId, likes, liked }: {
postId: string; likes: number; liked: boolean;
}) {
const [optimistic, addOptimistic] = useOptimistic(
{ likes, liked },
(state, next: boolean) => ({
liked: next,
likes: state.likes + (next ? 1 : -1),
})
);
const [, startTransition] = useTransition();
return (
<button
onClick={() =>
startTransition(async () => {
addOptimistic(!optimistic.liked);
await toggleLike(postId, !optimistic.liked);
})
}
>
{optimistic.liked ? "❤️" : "🤍"} {optimistic.likes}
</button>
);
}Why You Need a Transition
Optimistic updates must happen inside a transition. Calling addOptimistic outside one throws an error.
- Use
useTransitionfor button clicks, or - Use
<form action={...}>— form actions are wrapped in a transition automatically
The transition tells React the update is non-urgent and tied to a pending async action. When the action finishes (and revalidation arrives), the optimistic value is replaced by the real one.
Optimistic Comments with a Form
For adding a comment, the form's action already runs in a transition, so you can call addOptimistic directly inside it.
We append a temporary comment (with a sending flag) so the user sees it immediately, then submit to the server.
"use client";
import { useOptimistic, useRef } from "react";
import { addComment } from "./actions";
type Comment = { id: string; text: string; sending?: boolean };
export function Comments({ postId, comments }: {
postId: string; comments: Comment[];
}) {
const formRef = useRef<HTMLFormElement>(null);
const [optimistic, addOptimistic] = useOptimistic(
comments,
(state, text: string) => [
...state,
{ id: crypto.randomUUID(), text, sending: true },
]
);
return (
<>
<ul>
{optimistic.map((c) => (
<li key={c.id} style={{ opacity: c.sending ? 0.5 : 1 }}>
{c.text}
</li>
))}
</ul>
<form
ref={formRef}
action={async (formData) => {
const text = formData.get("text") as string;
addOptimistic(text);
formRef.current?.reset();
await addComment(postId, text);
}}
>
<input name="text" />
<button type="submit">Post</button>
</form>
</>
);
}The Mental Model: Snapshots, Not Mutations
useOptimistic does not change your real state. It layers a temporary view on top.
- The base state is your prop/state from the server.
- The optimistic value lives only while a transition is pending.
- When the transition settles, React throws the optimistic value away and re-renders from the (now updated) base state.
This is why rollback is free: there is nothing to undo, you simply stop showing the optimistic layer.
A Reusable Toggle Reducer
The update function is just a pure reducer. You can model any toggle — likes, bookmarks, follow/unfollow — by switching a boolean and deriving counts.
Because it is pure, you can unit-test it without React. Here is a standalone reducer that mirrors what we pass to useOptimistic.
type ToggleState = { active: boolean; count: number };
function toggleReducer(state: ToggleState, next: boolean): ToggleState {
return {
active: next,
count: state.count + (next ? 1 : -1),
};
}
const start: ToggleState = { active: false, count: 10 };
const afterLike = toggleReducer(start, true);
const afterUnlike = toggleReducer(afterLike, false);
console.log(afterLike); // { active: true, count: 11 }
console.log(afterUnlike); // { active: false, count: 10 }Handling Failures Gracefully
If the Server Action rejects, React reverts the optimistic state, but the user should know why. Wrap the action in try/catch and surface an error (a toast, an inline message).
The optimistic value disappears automatically; your job is only to communicate the failure.
startTransition(async () => {
addOptimistic(!optimistic.liked);
try {
await toggleLike(postId, !optimistic.liked);
} catch (err) {
// optimistic value already reverted by React
toast.error("Could not save your like. Try again.");
}
});Keep the Base State in Sync
For automatic reconciliation to work, the base state must come from the server and refresh after the mutation. Two common ways:
revalidatePath/revalidateTaginside the Server Action re-fetches the page data.- Passing fresh props down to the Client Component (e.g. from a Server Component parent).
If the base state never updates, the optimistic value will vanish and snap back to the old count — a classic flicker bug.
useOptimistic vs useState
You could fake optimism with useState, but you would have to manually save the previous value, write rollback logic, and clear it after the request. That is error-prone.
useOptimisticties the optimistic value to the transition lifecycle — auto-applied while pending, auto-discarded when settled.- It re-syncs whenever the base state changes, so server truth always wins.
Reach for useOptimistic whenever an action has a clear, predictable success outcome you can show immediately.
Quick Check
Test your understanding of how useOptimistic behaves.
Recap
You learned how to deliver instant feedback with useOptimistic in Next.js 15:
- useOptimistic(state, reducer) returns an optimistic view plus an
addOptimisticfunction. - Optimistic updates must run inside a transition (via
useTransitionor a formaction). - The real mutation lives in a Server Action that calls
revalidatePath/revalidateTagso the base state refreshes. - On success the optimistic layer is replaced by server truth; on failure React auto-reverts — you just surface an error.
- Prefer it over manual
useStaterollback for likes, comments, and toggles.
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Ist die Lektion „Sofortiges Feedback mit useOptimistic“ kostenlos?
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Was lerne ich in „Sofortiges Feedback mit useOptimistic“?
Wenden Sie optimistische UI-Aktualisierungen für Likes, Kommentare und Umschalter an, während die Action im Hintergrund abgeschlossen wird. Du übst Next.js 15 Fullstack (App Router + Server Actions) mit praktischem Code, den du direkt im Browser ausführst, und ein 24/7 KI-Tutor beantwortet deine Fragen während du die Lektion bearbeitest.
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Kann ich in dieser Next.js 15 Fullstack (App Router + Server Actions)-Lektion Code schreiben und ausführen?
Ja. Jede Next.js 15 Fullstack (App Router + Server Actions)-Lektion enthält einen integrierten Code-Editor, sodass du echten Code direkt in deinem Browser schreibst und ausführst und sofort KI-Feedback erhältst — ohne lokale Einrichtung erforderlich.
Alle Lektionen in diesem Kurs
- Progressive Enhancement mit der form-Action-Prop
- Ausstehende und Ladezustände mit useFormStatus
- Feldbezogene Validierungsfehler mit useActionState
- Sofortiges Feedback mit useOptimistic