Close the loop.
Ship the feature.

const [pokemon, setPokemon] = useState(null);
const [isLoading, setIsLoading] = useState(true);
const [error, setError] = useState(null);

useEffect(() => {
  async function fetchPokemon() {
    setIsLoading(true);
    setError(null);
    try {
      const res = await fetch(url);
      if (!res.ok) throw new Error("request failed");
      setPokemon(await res.json());
    } catch (e) {
      setError(e.message);
    } finally {
      setIsLoading(false);
    }
  }
  fetchPokemon();
}, [url]);

if (isLoading) return <Skeleton />;
if (error) return <ErrorState message={error} />;
if (!pokemon) return <Empty />;

return <PokemonCard data={pokemon} />;

// Better, but we still have one subtle async bug...

// User toggles between id=25 and id=133 quickly
const [id, setId] = useState<number>(25);
const [imageUrl, setImageUrl] = useState<string | null>(null);

useEffect(() => {
  async function load() {
    const res = await fetch(`/api/pokemon/${id}`);
    const data = await res.json();
    setImageUrl(data.sprites.front_default);
  }
  load();
}, [id]);

// t0: id=25 request starts (slow network)
// t1: user clicks next -> id=133 request starts
// t2: id=133 resolves first -> image becomes Eevee ✓
// t3: id=25 resolves later -> setImageUrl(Pikachu) ❌

// UI now flashes stale image even though id is 133.
// That's the race: late old response overwrites new state.

function App() {
  const [projectId, setProjectId] = React.useState("p_123");
  const [project, setProject] = React.useState(null);
  const [isLoading, setIsLoading] = React.useState(true);
  const [error, setError] = React.useState(null);

  React.useEffect(() => {
    let ignore = false;

    const loadProject = async () => {
      setProject(null);
      setIsLoading(true);
      setError(null);

      try {
        const projectRes = await fetch(`/api/projects/${projectId}`);
        if (ignore) return;
        if (!projectRes.ok) {
          throw new Error("Failed to load project");
        }

        setProject(await projectRes.json());
        setIsLoading(false);
      } catch (e) {
        setError(e.message);
        setIsLoading(false);
      }
    };

    loadProject();
    return () => { ignore = true; };
  }, [projectId]);

  if (isLoading) return <Skeleton />;
  if (error) return <ErrorState message={error} />;
  if (!project) return <Empty />;

  return <ProjectHeader project={project} />;
}

function ProjectPage({ projectId }) {
  const project = useQuery({
    queryKey: ["project", projectId],
    queryFn: () => fetchProject(projectId),
  });

  if (project.isPending) return <Skeleton />;
  if (project.error) return <ErrorState />;
  if (!project.data) return <Empty />;

  return <ProjectHeader project={project.data} />;
}

async function fetchProject(projectId) {
  const res = await fetch(`/api/projects/${projectId}`);
  if (!res.ok) throw new Error("Failed to load project");
  return res.json();
}

const project = useQuery({
  queryKey: ["project", projectId],
  queryFn: () => fetchProject(projectId),
});

if (project.isPending) return <Skeleton />;
if (project.error) return <ErrorState />;

return <ProjectHeaderView project={project.data} />;

Contract
- queryKey identifies one cache entry
- queryFn defines how that key is fetched

Behavior
- runs automatically when mounted
- serves cached data for same key
- background refetch on focus/reconnect/mount (default stale behavior)

Outcome
- multiple components with same key stay synchronized

const createTask = useMutation({
  mutationFn: createTaskApi,
  onSuccess: async (_data, vars) => {
    await queryClient.invalidateQueries({
      queryKey: ["tasks", vars.projectId],
    });
  },
});

1) Match cache entries by queryKey filter
2) Mark matched entries stale
3) Active observers refetch in background

Matching examples
- { queryKey: ["tasks"] } => all task queries
- { queryKey: ["tasks", projectId], exact: true } => one query

Result
- post-mutation UI converges back to server truth

const createTask = useMutation({
  mutationFn: createTaskApi,
});

const onSubmit = (input) => {
  createTask.mutate(input);
};

Trigger styles
- mutate(input): fire-and-forget callback flow
- mutateAsync(input): Promise flow with await/try-catch

States
- idle -> pending -> success | error

Key behavior
- stores last error/data for this mutation
- does not refetch queries by itself
- default retry is false (unlike queries)

const updateTask = useMutation({
  mutationFn: updateTaskApi,
  onSuccess: async (_data, vars) => {
    await Promise.all([
      queryClient.invalidateQueries({
        queryKey: ["tasks", vars.projectId],
      }),
      queryClient.invalidateQueries({
        queryKey: ["projectStats", vars.projectId],
      }),
    ]);
  },
});

const updateTask = useMutation({
  mutationFn: updateTaskApi,
  onSuccess: (updatedTask, vars) => {
    queryClient.setQueryData(
      ["task", updatedTask.id],
      updatedTask
    );

    queryClient.setQueryData(
      ["tasks", vars.projectId],
      (old = []) =>
        old.map((t) => t.id === updatedTask.id ? updatedTask : t)
    );
  },
});

import { api } from "~/trpc/react";

export function TaskList() {
  const { data, isLoading, error }
    = api.task.getAll.useQuery();

  if (isLoading) return <Spinner />;
  if (error) return <Error />;

  return (
    <ul>
      {data.map(task => (
        <li key={task.id}>
          {task.title}
          {/* ^ fully typed */}
        </li>
      ))}
    </ul>
  );
}

export function CreateTask() {
  const utils = api.useUtils();

  const create = api.task.create
    .useMutation({
      onSuccess: () => {
        // Invalidate stale data
        utils.task.getAll.invalidate();
      },
    });

  const handleSubmit = () => {
    create.mutate({
      title: "New Task",
      priority: "MEDIUM",
      projectId: "...",
    });
  };

  return (
    <button onClick={handleSubmit}>
      Add Task
    </button>
  );
}

<div>
  <h2>My Tasks</h2>
  <ul>
    <li>Fix login bug</li>
    <li>Add search</li>
  </ul>
</div>

<div className="max-w-md mx-auto p-6">
  <h2 className="text-2xl
    font-bold mb-4">
    My Tasks
  </h2>
  <ul className="space-y-2">
    <li className="p-4 rounded-lg
      bg-gray-800 shadow
      hover:bg-gray-700
      transition">
      Fix login bug
    </li>
  </ul>
</div>