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Next.js 15 Fullstack (App Router + Server Actions) · Lección

Respuesta instantánea con useOptimistic

Aplique actualizaciones optimistas de la interfaz para Me gusta, comentarios y conmutadores mientras la acción se resuelve en segundo plano.

Respuesta instantánea con useOptimistic es una lección gratuita de Next.js 15 Fullstack (App Router + Server Actions) en CoddyKit. Esta es la lección 4 de 4. Puedes leer la lección completa abajo gratuitamente — luego la practicas en el navegador con un editor de código integrado y un tutor de IA 24/7. Forma parte de la ruta de aprendizaje de Next.js 15 Fullstack (App Router + Server Actions), y tu progreso se sincroniza en la web y la app de CoddyKit. El curso de Next.js 15 Fullstack (App Router + Server Actions) incluye 4 lecciones en total.

Partes de esta lección aún no han sido traducidas y se muestran en inglés.

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.

Preguntas frecuentes

¿La lección «Respuesta instantánea con useOptimistic» es gratis?

Sí — el texto completo de «Respuesta instantánea con useOptimistic» es gratis para leer aquí en la web. Para practicarla de forma interactiva (editor de código integrado y tutor de IA 24/7) y desbloquear el resto del curso de Next.js 15 Fullstack (App Router + Server Actions), actualiza a CoddyKit PRO. El curso de Next.js 15 Fullstack (App Router + Server Actions) incluye 4 lecciones en total.

¿Qué aprenderé en «Respuesta instantánea con useOptimistic»?

Aplique actualizaciones optimistas de la interfaz para Me gusta, comentarios y conmutadores mientras la acción se resuelve en segundo plano. Practicas Next.js 15 Fullstack (App Router + Server Actions) con código real que ejecutas directamente en el navegador, y un tutor de IA 24/7 responde tus preguntas mientras trabajas en la lección.

¿Necesito experiencia previa para empezar Next.js 15 Fullstack (App Router + Server Actions)?

No se requiere experiencia previa. Next.js 15 Fullstack (App Router + Server Actions) en CoddyKit está estructurado para principiantes hasta estudiantes avanzados, así que puedes empezar aquí o desde el inicio y avanzar a tu ritmo. Esta es la lección 4 de 4.

¿Cuánto tiempo toma la lección «Respuesta instantánea con useOptimistic»?

La mayoría de las lecciones de CoddyKit toman alrededor de 5–10 minutos. Cada una es compacta e interactiva, así que avanzas constantemente y retomas exactamente por donde dejaste en la web y la app.

¿Puedo escribir y ejecutar código en esta lección de Next.js 15 Fullstack (App Router + Server Actions)?

Sí. Cada lección de Next.js 15 Fullstack (App Router + Server Actions) incluye un editor de código integrado, así que escribes y ejecutas código real directamente en tu navegador y obtienes retroalimentación instantánea de IA — sin configuración local necesaria.

Todas las lecciones de este curso

  1. Mejora progresiva con la propiedad Action de form
  2. Estados de espera y carga con useFormStatus
  3. Errores de validación por campo con useActionState
  4. Respuesta instantánea con useOptimistic
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