SolidStart Optimistic UI

Optimistic UI shows the expected result immediately without waiting for the server response. If the request fails, the UI rolls back to the previous state.

Core Concept

Use

useSubmissions

to track pending action submissions and combine them with server data to create an optimistic view.

Basic Pattern

import {
  action,
  createAsync,
  query,
  useAction,
  useSubmissions,
} from "@solidjs/router";
import { For } from "solid-js";

// Server data
const getServerTodos = query(async () => {
  "use server";
  return todos;
}, "serverTodos");

// Mutation action
const addTodo = action(async (title: string) => {
  "use server";
  await new Promise(r => setTimeout(r, 2000));
  todos.push({ title });
}, "add-todo");

export default function Home() {
  const serverTodos = createAsync(() => getServerTodos());
  const addTodoAction = useAction(addTodo);
  const todoSubmissions = useSubmissions(addTodo);

  // Extract pending todos from submissions
  const pendingTodos = () =>
    todoSubmissions
      .filter(submission => submission.pending)
      .map(submission => ({
        title: submission.input[0] + " (pending)",
      }));

  // Combine server data with pending items
  const todos = () =>
    serverTodos() ? [...serverTodos(), ...pendingTodos()] : [];

  return (
    <main>
      <h1>Todos</h1>
      <button onClick={() => addTodoAction("Todo 3")}>Add Todo</button>
      <For each={todos()}>
        {(todo) => <h3>{todo.title}</h3>}
      </For>
    </main>
  );
}

Key Steps

1. Get submissions: Use

   useSubmissions(action)

   to track all submissions for an action
2. Filter pending: Use

   .filter(submission => submission.pending)

   to only show in-flight requests
3. Map to data shape: Transform

   submission.input

   into the same shape as server data
4. Combine: Merge server data with pending items:

   [...serverData, ...pendingItems]

Important Notes

Accessing Submissions Values

useSubmissions

returns a reactive array (with a

.pending

boolean). Use it directly:

// ✅ Correct
const pendingTodos = () =>
  todoSubmissions.filter(submission => submission.pending);

// ✅ Also valid
const pendingTodos = () => [...todoSubmissions];

Filter by Pending State

Only include pending submissions to avoid duplicates. Once a submission completes, the server data will include it:

const pendingTodos = () =>
  todoSubmissions
    .filter(submission => submission.pending) // Only pending ones!
    .map(submission => ({ title: submission.input[0] }));

Accessing Action Input

The

submission.input

array contains the arguments passed to the action:

// Action with single argument
const addTodo = action(async (title: string) => { ... }, "add-todo");
// submission.input[0] is the title

// Action with multiple arguments
const updateTodo = action(async (id: string, title: string) => { ... }, "update-todo");
// submission.input[0] is id, submission.input[1] is title

// Action with FormData
const addPost = action(async (formData: FormData) => { ... }, "add-post");
// submission.input[0] is FormData
const title = submission.input[0].get("title");

Complete Example with FormData

import {
  action,
  createAsync,
  query,
  useSubmissions,
} from "@solidjs/router";
import { For } from "solid-js";

const getPosts = query(async () => {
  "use server";
  return posts;
}, "posts");

const addPost = action(async (formData: FormData) => {
  "use server";
  const title = formData.get("title") as string;
  await new Promise(r => setTimeout(r, 1500));
  posts.push({ title });
}, "add-post");

export default function Home() {
  const serverPosts = createAsync(() => getPosts());
  const postSubmissions = useSubmissions(addPost);

  const pendingPosts = () =>
    postSubmissions
      .filter(submission => submission.pending)
      .map(submission => {
        const formData = submission.input[0] as FormData;
        return {
          title: formData.get("title") + " (pending)",
        };
      });

  const posts = () =>
    serverPosts() ? [...serverPosts(), ...pendingPosts()] : [];

  return (
    <main>
      <form action={addPost} method="post">
        <input name="title" />
        <button>Add Post</button>
      </form>
      <For each={posts()}>
        {(post) => <article>{post.title}</article>}
      </For>
    </main>
  );
}

With Multiple Arguments

const updateTodo = action(async (id: string, title: string, completed: boolean) => {
  "use server";
  await updateTodoInDB(id, { title, completed });
}, "update-todo");

export default function TodoList() {
  const todos = createAsync(() => getTodos());
  const submissions = useSubmissions(updateTodo);

  const pendingUpdates = () =>
    submissions
      .filter(submission => submission.pending)
      .map(submission => ({
        id: submission.input[0],      // id
        title: submission.input[1],   // title
        completed: submission.input[2], // completed
        pending: true,
      }));

  // Merge: replace existing todos with pending updates, then add new ones
  const todosWithPending = () => {
    const serverData = todos() || [];
    const pending = pendingUpdates();
    
    return serverData.map(todo => {
      const pendingUpdate = pending.find(p => p.id === todo.id);
      return pendingUpdate || todo;
    }).concat(
      pending.filter(p => !serverData.some(t => t.id === p.id))
    );
  };

  return <For each={todosWithPending()}>...</For>;
}

Benefits

1. Instant feedback: Users see changes immediately
2. Better UX: No waiting for network requests
3. Automatic cleanup: When submission completes, it's no longer pending, so it disappears from the optimistic list and appears in server data
4. No manual state management: Solid's reactivity handles updates automatically

Best Practices

1. Filter by pending: Always filter to only show pending submissions to avoid duplicates
2. Match data shape: Ensure optimistic items match the shape of server data
3. Visual indicators: Consider adding visual indicators (e.g., "(pending)") to distinguish optimistic items
4. Handle errors: If an action fails, the submission will no longer be pending, so it automatically disappears from the optimistic list
5. Use with server actions: This pattern works best with SolidStart actions, not just client-side state

When to Use

• Adding items to a list
• Updating item properties
• Toggling boolean states (like, favorite, completed)
• Any mutation where you can predict the result

When NOT to Use

• When the server response contains data you can't predict (e.g., generated IDs, timestamps)
• When operations are likely to fail validation
• When immediate consistency is critical