Next.js 15 Fullstack (App Router + Server Actions) · レッスン

useOptimisticによる即時フィードバック

アクションがバックグラウンドで完了する間、いいね、コメント、切り替えに楽観的UI更新を適用します。

「useOptimisticによる即時フィードバック」はCoddyKit上の無料Next.js 15 Fullstack (App Router + Server Actions)レッスンです。これはレッスン4/4です。下記で完全なレッスンを無料で読むことができます。その後、ブラウザ内の組み込みコードエディタと24時間対応のAIチューターでハンズオン演習できます。これはNext.js 15 Fullstack (App Router + Server Actions)学習パスの一部であり、ウェブとCoddyKitアプリ全体で進捗が同期されます。 Next.js 15 Fullstack (App Router + Server Actions)コースには全4レッスンが含まれています。

このレッスンの一部はまだ翻訳されておらず、英語で表示されています。

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 now
addOptimistic(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 useTransition for 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 / revalidateTag inside 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.

useOptimistic ties 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 addOptimistic function.
Optimistic updates must run inside a transition (via useTransition or a form action).
The real mutation lives in a Server Action that calls revalidatePath/revalidateTag so 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 useState rollback for likes, comments, and toggles.

よくある質問

「useOptimisticによる即時フィードバック」レッスンは無料ですか？

はい。「useOptimisticによる即時フィードバック」の完全なテキストはこのウェブで無料で読めます。インタラクティブに演習し（組み込みコードエディタと24時間対応のAIチューター）、Next.js 15 Fullstack (App Router + Server Actions)コースの残りをアンロックするには、CoddyKit PROにアップグレードしてください。 Next.js 15 Fullstack (App Router + Server Actions)コースには全4レッスンが含まれています。

「useOptimisticによる即時フィードバック」で何を学びますか？

アクションがバックグラウンドで完了する間、いいね、コメント、切り替えに楽観的UI更新を適用します。ブラウザで直接実行するハンズオンコードでNext.js 15 Fullstack (App Router + Server Actions)を演習し、24時間対応のAIチューターがレッスンを進める中での質問に答えます。

Next.js 15 Fullstack (App Router + Server Actions)を始めるのに経験は必要ですか？

事前経験は必要ありません。CoddyKitのNext.js 15 Fullstack (App Router + Server Actions)は初級者から上級者向けに構成されているため、ここから始めるか最初から始めて、自分のペースで進むことができます。これはレッスン4/4です。

「useOptimisticによる即時フィードバック」レッスンにはどのくらい時間がかかりますか？

ほとんどのCoddyKitレッスンは約5～10分かかります。各レッスンはコンパクトでインタラクティブなので、着実に進歩し、ウェブとアプリ全体で正確に前回の場所から再開できます。

このNext.js 15 Fullstack (App Router + Server Actions)レッスンでコードを書いて実行できますか？

はい。すべてのNext.js 15 Fullstack (App Router + Server Actions)レッスンに組み込みコードエディタが含まれているため、ブラウザでリアルコードを書いて実行し、即座のAIフィードバックを取得できます。ローカル設定は不要です。

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