Optimistic UI

Optimistic mutation pattern for list add and remove actions with rollback on simulated failures.

vanilla-js css react vue svelte

Targets: TS JS HTML React Vue Svelte

Code

* {
  box-sizing: border-box;
}

:root {
  --bg: #060a13;
  --panel: #111827;
  --text: #e2e8f0;
  --muted: #94a3b8;
  --border: rgba(255, 255, 255, 0.14);
  --accent: #22d3ee;
  --danger: #fb7185;
}

body {
  margin: 0;
  background: linear-gradient(165deg, #0f1c35, var(--bg));
  color: var(--text);
  font-family: "Sora", system-ui, sans-serif;
}

.shell {
  width: min(640px, calc(100% - 2rem));
  margin: 2rem auto;
}

h1 {
  margin-bottom: 0.35rem;
}

p {
  margin-top: 0;
  color: var(--muted);
}

.task-form {
  display: flex;
  gap: 0.6rem;
  margin-bottom: 0.8rem;
}

.task-form input {
  flex: 1;
  border: 1px solid var(--border);
  border-radius: 10px;
  background: var(--panel);
  color: var(--text);
  padding: 0.6rem;
}

.task-form button {
  border: 0;
  border-radius: 10px;
  padding: 0.6rem 0.9rem;
  font-weight: 700;
  background: var(--accent);
  color: #032026;
}

.status {
  min-height: 1.25rem;
  margin: 0 0 0.8rem;
  font-size: 0.85rem;
}

.tasks {
  margin: 0;
  padding: 0;
  list-style: none;
  display: grid;
  gap: 0.6rem;
}

.tasks li {
  border: 1px solid var(--border);
  border-radius: 12px;
  background: var(--panel);
  padding: 0.75rem;
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 0.7rem;
}

.task-left {
  display: flex;
  align-items: center;
  gap: 0.55rem;
}

.pending {
  font-size: 0.75rem;
  padding: 0.1rem 0.4rem;
  border-radius: 999px;
  border: 1px solid #67e8f9;
  color: #a5f3fc;
}

.tasks button {
  border: 1px solid rgba(251, 113, 133, 0.4);
  background: rgba(251, 113, 133, 0.12);
  color: #fecdd3;
  border-radius: 8px;
  padding: 0.35rem 0.6rem;
}

.status.error {
  color: var(--danger);
}

.status.ok {
  color: #67e8f9;
}

(() => {
  let tasks = [
    { id: 1, label: "Ship release notes", pending: false },
    { id: 2, label: "Clean API logs", pending: false },
  ];

  const list = document.getElementById("tasks");
  const form = document.getElementById("task-form");
  const input = document.getElementById("task-input");
  const status = document.getElementById("status");

  let nextId = 3;

  const setStatus = (message, tone = "ok") => {
    status.textContent = message;
    status.className = `status ${tone}`;
  };

  const fakeRequest = () =>
    new Promise((resolve, reject) => {
      const delay = 300 + Math.random() * 700;
      setTimeout(() => {
        if (Math.random() < 0.25) {
          reject(new Error("Request failed"));
        } else {
          resolve(true);
        }
      }, delay);
    });

  const render = () => {
    list.innerHTML = "";

    for (const task of tasks) {
      const li = document.createElement("li");
      li.innerHTML = `
        <div class="task-left">
          <span>${task.label}</span>
          ${task.pending ? '<span class="pending">pending</span>' : ""}
        </div>
        <button type="button" data-delete="${task.id}">Delete</button>
      `;
      list.appendChild(li);
    }
  };

  form.addEventListener("submit", async (event) => {
    event.preventDefault();
    const label = input.value.trim();
    if (!label) return;

    const tempId = -Date.now();
    tasks = [{ id: tempId, label, pending: true }, ...tasks];
    input.value = "";
    setStatus("Task added optimistically...");
    render();

    try {
      await fakeRequest();
      tasks = tasks.map((task) => {
        if (task.id !== tempId) return task;
        return { id: nextId++, label: task.label, pending: false };
      });
      setStatus("Task confirmed by server.", "ok");
    } catch (_error) {
      tasks = tasks.filter((task) => task.id !== tempId);
      setStatus("Add failed. Rolled back.", "error");
    }

    render();
  });

  list.addEventListener("click", async (event) => {
    const target = event.target;
    if (!(target instanceof HTMLElement)) return;
    const value = target.getAttribute("data-delete");
    if (!value) return;

    const id = Number(value);
    const index = tasks.findIndex((task) => task.id === id);
    if (index < 0) return;

    const [removed] = tasks.splice(index, 1);
    render();
    setStatus("Task removed optimistically...");

    try {
      await fakeRequest();
      setStatus("Deletion confirmed by server.", "ok");
    } catch (_error) {
      tasks.splice(index, 0, removed);
      setStatus("Delete failed. Restored item.", "error");
      render();
    }
  });

  render();
})();

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8" />
  <meta name="viewport" content="width=device-width, initial-scale=1.0" />
  <title>Optimistic UI</title>
  <link rel="stylesheet" href="style.css" />
</head>
<body>
  <main class="shell">
    <h1>Optimistic UI</h1>
    <p>Immediate UI updates with rollback when a request fails.</p>

    <form id="task-form" class="task-form">
      <input id="task-input" type="text" placeholder="Add task" aria-label="Add task" />
      <button type="submit">Add</button>
    </form>

    <p id="status" class="status" role="status" aria-live="polite"></p>

    <ul id="tasks" class="tasks"></ul>
  </main>

  <script src="script.js"></script>
</body>
</html>

import { type FormEvent, useRef, useState } from "react";

type Task = {
  id: number;
  label: string;
  pending: boolean;
};

type NoticeTone = "idle" | "info" | "ok" | "error";

const fakeRequest = () =>
  new Promise<void>((resolve, reject) => {
    const delay = 280 + Math.random() * 720;
    setTimeout(() => {
      if (Math.random() < 0.28) {
        reject(new Error("Request failed"));
      } else {
        resolve();
      }
    }, delay);
  });

export default function OptimisticUiPattern() {
  const [tasks, setTasks] = useState<Task[]>([
    { id: 1, label: "Ship release notes", pending: false },
    { id: 2, label: "Clean API logs", pending: false },
  ]);
  const [value, setValue] = useState("");
  const [notice, setNotice] = useState<{ message: string; tone: NoticeTone }>({
    message: "Use optimistic updates to keep interactions instant.",
    tone: "info",
  });

  const nextIdRef = useRef(3);
  const nextTempIdRef = useRef(-1);

  const onAdd = async (event: FormEvent) => {
    event.preventDefault();
    const label = value.trim();
    if (!label) {
      setNotice({ message: "Enter a task name before adding.", tone: "error" });
      return;
    }

    const tempId = nextTempIdRef.current;
    nextTempIdRef.current -= 1;

    setValue("");
    setTasks((prev) => [{ id: tempId, label, pending: true }, ...prev]);
    setNotice({ message: "Task added optimistically. Syncing with server...", tone: "info" });

    try {
      await fakeRequest();
      const confirmedId = nextIdRef.current;
      nextIdRef.current += 1;

      setTasks((prev) =>
        prev.map((task) =>
          task.id === tempId ? { ...task, id: confirmedId, pending: false } : task
        )
      );
      setNotice({ message: "Task confirmed by server.", tone: "ok" });
    } catch {
      setTasks((prev) => prev.filter((task) => task.id !== tempId));
      setNotice({ message: "Add failed. Optimistic task was rolled back.", tone: "error" });
    }
  };

  const onDelete = async (id: number) => {
    let removed: Task | null = null;
    let removedIndex = -1;

    setTasks((prev) => {
      removedIndex = prev.findIndex((task) => task.id === id);
      if (removedIndex === -1) return prev;
      removed = prev[removedIndex];
      return prev.filter((task) => task.id !== id);
    });

    if (!removed) return;
    const rollbackTask = removed;
    const rollbackIndex = removedIndex;

    setNotice({ message: "Task removed optimistically. Syncing with server...", tone: "info" });

    try {
      await fakeRequest();
      setNotice({ message: "Deletion confirmed by server.", tone: "ok" });
    } catch {
      setTasks((prev) => {
        if (prev.some((task) => task.id === rollbackTask.id)) return prev;
        const next = prev.slice();
        const safeIndex = Math.min(Math.max(rollbackIndex, 0), next.length);
        next.splice(safeIndex, 0, rollbackTask);
        return next;
      });
      setNotice({ message: "Delete failed. Item restored.", tone: "error" });
    }
  };

  const noticeClass =
    notice.tone === "ok"
      ? "text-emerald-300"
      : notice.tone === "error"
        ? "text-rose-300"
        : "text-sky-300";

  return (
    <section className="min-h-screen bg-[#0d1117] px-4 py-6 text-[#e6edf3]">
      <div className="mx-auto max-w-2xl space-y-4">
        <header className="rounded-2xl border border-[#30363d] bg-[#161b22] p-4">
          <p className="text-xs font-bold uppercase tracking-wide text-[#8b949e]">Pattern</p>
          <h1 className="mt-1 text-lg font-bold">Optimistic UI</h1>
          <p className="mt-1 text-sm text-[#8b949e]">
            UI updates immediately and rolls back only if the request fails.
          </p>
        </header>

        <form
          onSubmit={onAdd}
          className="flex gap-2 rounded-2xl border border-[#30363d] bg-[#161b22] p-3"
        >
          <input
            value={value}
            onChange={(event) => setValue(event.target.value)}
            placeholder="Add task"
            className="flex-1 rounded-lg border border-[#30363d] bg-[#0d1117] px-3 py-2 text-sm text-[#e6edf3] placeholder-[#6b7280] outline-none transition-colors focus:border-[#58a6ff]"
          />
          <button
            type="submit"
            className="rounded-lg border border-[#58a6ff]/45 bg-[#58a6ff]/15 px-4 py-2 text-sm font-semibold text-[#c9e6ff] transition-colors hover:bg-[#58a6ff]/25"
          >
            Add
          </button>
        </form>

        <p className={`min-h-5 text-xs ${noticeClass}`}>{notice.message}</p>

        <ul className="grid gap-2">
          {tasks.map((task) => (
            <li
              key={task.id}
              className="flex items-center justify-between gap-3 rounded-xl border border-[#30363d] bg-[#161b22] px-3 py-2.5"
            >
              <div className="flex items-center gap-2 text-sm">
                <span className={task.pending ? "text-[#cdd9e5]" : "text-[#e6edf3]"}>
                  {task.label}
                </span>
                {task.pending && (
                  <span className="rounded-full border border-sky-300/30 bg-sky-400/10 px-2 py-0.5 text-[10px] font-semibold uppercase tracking-wide text-sky-200">
                    syncing
                  </span>
                )}
              </div>
              <button
                type="button"
                disabled={task.pending}
                onClick={() => onDelete(task.id)}
                className="rounded-md border border-rose-500/35 bg-rose-500/10 px-2.5 py-1 text-xs font-semibold text-rose-300 transition-colors enabled:hover:bg-rose-500/20 disabled:cursor-not-allowed disabled:opacity-50"
              >
                Delete
              </button>
            </li>
          ))}
          {tasks.length === 0 && (
            <li className="rounded-xl border border-dashed border-[#30363d] bg-[#161b22] px-3 py-5 text-center text-sm text-[#8b949e]">
              No tasks left. Add one to test optimistic create.
            </li>
          )}
        </ul>
      </div>
    </section>
  );
}

<script setup>
import { ref, computed } from "vue";

const tasks = ref([
  { id: 1, label: "Ship release notes", pending: false },
  { id: 2, label: "Clean API logs", pending: false },
]);
const value = ref("");
const notice = ref({
  message: "Use optimistic updates to keep interactions instant.",
  tone: "info",
});
let nextId = 3;
let nextTempId = -1;

function fakeRequest() {
  return new Promise((resolve, reject) => {
    const delay = 280 + Math.random() * 720;
    setTimeout(() => {
      if (Math.random() < 0.28) reject(new Error("Request failed"));
      else resolve();
    }, delay);
  });
}

async function onAdd() {
  const label = value.value.trim();
  if (!label) {
    notice.value = { message: "Enter a task name before adding.", tone: "error" };
    return;
  }

  const tempId = nextTempId;
  nextTempId -= 1;

  value.value = "";
  tasks.value = [{ id: tempId, label, pending: true }, ...tasks.value];
  notice.value = { message: "Task added optimistically. Syncing with server...", tone: "info" };

  try {
    await fakeRequest();
    const confirmedId = nextId;
    nextId += 1;
    tasks.value = tasks.value.map((t) =>
      t.id === tempId ? { ...t, id: confirmedId, pending: false } : t
    );
    notice.value = { message: "Task confirmed by server.", tone: "ok" };
  } catch {
    tasks.value = tasks.value.filter((t) => t.id !== tempId);
    notice.value = { message: "Add failed. Optimistic task was rolled back.", tone: "error" };
  }
}

async function onDelete(id) {
  const removedIndex = tasks.value.findIndex((t) => t.id === id);
  if (removedIndex === -1) return;
  const removed = tasks.value[removedIndex];

  tasks.value = tasks.value.filter((t) => t.id !== id);
  notice.value = { message: "Task removed optimistically. Syncing with server...", tone: "info" };

  try {
    await fakeRequest();
    notice.value = { message: "Deletion confirmed by server.", tone: "ok" };
  } catch {
    if (!tasks.value.some((t) => t.id === removed.id)) {
      const next = [...tasks.value];
      const safeIndex = Math.min(Math.max(removedIndex, 0), next.length);
      next.splice(safeIndex, 0, removed);
      tasks.value = next;
    }
    notice.value = { message: "Delete failed. Item restored.", tone: "error" };
  }
}

const noticeClass = computed(() =>
  notice.value.tone === "ok"
    ? "text-emerald-300"
    : notice.value.tone === "error"
      ? "text-rose-300"
      : "text-sky-300"
);
</script>

<template>
  <section class="min-h-screen bg-[#0d1117] px-4 py-6 text-[#e6edf3]">
    <div class="mx-auto max-w-2xl space-y-4">
      <header class="rounded-2xl border border-[#30363d] bg-[#161b22] p-4">
        <p class="text-xs font-bold uppercase tracking-wide text-[#8b949e]">Pattern</p>
        <h1 class="mt-1 text-lg font-bold">Optimistic UI</h1>
        <p class="mt-1 text-sm text-[#8b949e]">
          UI updates immediately and rolls back only if the request fails.
        </p>
      </header>

      <form
        @submit.prevent="onAdd"
        class="flex gap-2 rounded-2xl border border-[#30363d] bg-[#161b22] p-3"
      >
        <input
          v-model="value"
          placeholder="Add task"
          class="flex-1 rounded-lg border border-[#30363d] bg-[#0d1117] px-3 py-2 text-sm text-[#e6edf3] placeholder-[#6b7280] outline-none transition-colors focus:border-[#58a6ff]"
        />
        <button
          type="submit"
          class="rounded-lg border border-[#58a6ff]/45 bg-[#58a6ff]/15 px-4 py-2 text-sm font-semibold text-[#c9e6ff] transition-colors hover:bg-[#58a6ff]/25"
        >
          Add
        </button>
      </form>

      <p :class="['min-h-5 text-xs', noticeClass]">{{ notice.message }}</p>

      <ul class="grid gap-2">
        <li
          v-for="task in tasks"
          :key="task.id"
          class="flex items-center justify-between gap-3 rounded-xl border border-[#30363d] bg-[#161b22] px-3 py-2.5"
        >
          <div class="flex items-center gap-2 text-sm">
            <span :class="task.pending ? 'text-[#cdd9e5]' : 'text-[#e6edf3]'">{{ task.label }}</span>
            <span v-if="task.pending" class="rounded-full border border-sky-300/30 bg-sky-400/10 px-2 py-0.5 text-[10px] font-semibold uppercase tracking-wide text-sky-200">
              syncing
            </span>
          </div>
          <button
            type="button"
            :disabled="task.pending"
            @click="onDelete(task.id)"
            class="rounded-md border border-rose-500/35 bg-rose-500/10 px-2.5 py-1 text-xs font-semibold text-rose-300 transition-colors enabled:hover:bg-rose-500/20 disabled:cursor-not-allowed disabled:opacity-50"
          >
            Delete
          </button>
        </li>
        <li v-if="tasks.length === 0" class="rounded-xl border border-dashed border-[#30363d] bg-[#161b22] px-3 py-5 text-center text-sm text-[#8b949e]">
          No tasks left. Add one to test optimistic create.
        </li>
      </ul>
    </div>
  </section>
</template>

<script>
let tasks = [
  { id: 1, label: "Ship release notes", pending: false },
  { id: 2, label: "Clean API logs", pending: false },
];
let value = "";
let notice = { message: "Use optimistic updates to keep interactions instant.", tone: "info" };
let nextId = 3;
let nextTempId = -1;

function fakeRequest() {
  return new Promise((resolve, reject) => {
    const delay = 280 + Math.random() * 720;
    setTimeout(() => {
      if (Math.random() < 0.28) reject(new Error("Request failed"));
      else resolve();
    }, delay);
  });
}

async function onAdd(event) {
  event.preventDefault();
  const label = value.trim();
  if (!label) {
    notice = { message: "Enter a task name before adding.", tone: "error" };
    return;
  }

  const tempId = nextTempId;
  nextTempId -= 1;

  value = "";
  tasks = [{ id: tempId, label, pending: true }, ...tasks];
  notice = { message: "Task added optimistically. Syncing with server...", tone: "info" };

  try {
    await fakeRequest();
    const confirmedId = nextId;
    nextId += 1;
    tasks = tasks.map((t) => (t.id === tempId ? { ...t, id: confirmedId, pending: false } : t));
    notice = { message: "Task confirmed by server.", tone: "ok" };
  } catch {
    tasks = tasks.filter((t) => t.id !== tempId);
    notice = { message: "Add failed. Optimistic task was rolled back.", tone: "error" };
  }
}

async function onDelete(id) {
  const removedIndex = tasks.findIndex((t) => t.id === id);
  if (removedIndex === -1) return;
  const removed = tasks[removedIndex];

  tasks = tasks.filter((t) => t.id !== id);
  notice = { message: "Task removed optimistically. Syncing with server...", tone: "info" };

  try {
    await fakeRequest();
    notice = { message: "Deletion confirmed by server.", tone: "ok" };
  } catch {
    if (!tasks.some((t) => t.id === removed.id)) {
      const next = [...tasks];
      const safeIndex = Math.min(Math.max(removedIndex, 0), next.length);
      next.splice(safeIndex, 0, removed);
      tasks = next;
    }
    notice = { message: "Delete failed. Item restored.", tone: "error" };
  }
}

$: noticeClass =
  notice.tone === "ok"
    ? "text-emerald-300"
    : notice.tone === "error"
      ? "text-rose-300"
      : "text-sky-300";
</script>

<section class="min-h-screen bg-[#0d1117] px-4 py-6 text-[#e6edf3]">
  <div class="mx-auto max-w-2xl space-y-4">
    <header class="rounded-2xl border border-[#30363d] bg-[#161b22] p-4">
      <p class="text-xs font-bold uppercase tracking-wide text-[#8b949e]">Pattern</p>
      <h1 class="mt-1 text-lg font-bold">Optimistic UI</h1>
      <p class="mt-1 text-sm text-[#8b949e]">
        UI updates immediately and rolls back only if the request fails.
      </p>
    </header>

    <form
      on:submit|preventDefault={onAdd}
      class="flex gap-2 rounded-2xl border border-[#30363d] bg-[#161b22] p-3"
    >
      <input
        bind:value={value}
        placeholder="Add task"
        class="flex-1 rounded-lg border border-[#30363d] bg-[#0d1117] px-3 py-2 text-sm text-[#e6edf3] placeholder-[#6b7280] outline-none transition-colors focus:border-[#58a6ff]"
      />
      <button
        type="submit"
        class="rounded-lg border border-[#58a6ff]/45 bg-[#58a6ff]/15 px-4 py-2 text-sm font-semibold text-[#c9e6ff] transition-colors hover:bg-[#58a6ff]/25"
      >
        Add
      </button>
    </form>

    <p class="min-h-5 text-xs {noticeClass}">{notice.message}</p>

    <ul class="grid gap-2">
      {#each tasks as task (task.id)}
        <li class="flex items-center justify-between gap-3 rounded-xl border border-[#30363d] bg-[#161b22] px-3 py-2.5">
          <div class="flex items-center gap-2 text-sm">
            <span class={task.pending ? "text-[#cdd9e5]" : "text-[#e6edf3]"}>{task.label}</span>
            {#if task.pending}
              <span class="rounded-full border border-sky-300/30 bg-sky-400/10 px-2 py-0.5 text-[10px] font-semibold uppercase tracking-wide text-sky-200">
                syncing
              </span>
            {/if}
          </div>
          <button
            type="button"
            disabled={task.pending}
            on:click={() => onDelete(task.id)}
            class="rounded-md border border-rose-500/35 bg-rose-500/10 px-2.5 py-1 text-xs font-semibold text-rose-300 transition-colors enabled:hover:bg-rose-500/20 disabled:cursor-not-allowed disabled:opacity-50"
          >
            Delete
          </button>
        </li>
      {/each}
      {#if tasks.length === 0}
        <li class="rounded-xl border border-dashed border-[#30363d] bg-[#161b22] px-3 py-5 text-center text-sm text-[#8b949e]">
          No tasks left. Add one to test optimistic create.
        </li>
      {/if}
    </ul>
  </div>
</section>

Optimistic UI

A resilient pattern for fast-feeling interfaces where UI updates immediately before server confirmation.

Features

Immediate optimistic insertion and deletion
Pending state tags
Simulated network latency
Automatic rollback on failure with status message