snapshot of https://github.com/TanStack/query.git (main, 2026-09-16; blobs > 131072B omitted)

examples/react/nextjs-app-optimistic-updates/components/TodoListCache.tsx

@@ -0,0 +1,130 @@
+'use client'
+
+import React, { useState } from 'react'
+import { useQuery, useMutation, useQueryClient } from '@tanstack/react-query'
+import type { Todo } from '@/app/api/todos/data'
+
+interface MutationContext {
+  previousTodos: Array<Todo> | undefined
+  optimisticId: string
+}
+
+async function fetchTodos(): Promise<Array<Todo>> {
+  const res = await fetch('/api/todos')
+  if (!res.ok) throw new Error('Failed to fetch todos')
+  return res.json()
+}
+
+async function addTodo(text: string): Promise<Todo> {
+  const res = await fetch('/api/todos', {
+    method: 'POST',
+    headers: { 'Content-Type': 'application/json' },
+    body: JSON.stringify({ text }),
+  })
+  if (!res.ok) {
+    const err = (await res.json()) as { error: string }
+    throw new Error(err.error)
+  }
+  return res.json()
+}
+
+export default function TodoListCache() {
+  const queryClient = useQueryClient()
+  const [inputValue, setInputValue] = useState('')
+  const [lastError, setLastError] = useState<string | null>(null)
+
+  const { data: todos = [] } = useQuery({
+    queryKey: ['todos'],
+    queryFn: fetchTodos,
+  })
+
+  const addTodoMutation = useMutation<Todo, Error, string, MutationContext>({
+    mutationFn: addTodo,
+    onMutate: async (text) => {
+      setLastError(null)
+      await queryClient.cancelQueries({ queryKey: ['todos'] })
+
+      const previousTodos = queryClient.getQueryData<Array<Todo>>(['todos'])
+
+      const optimisticId = `optimistic-${Date.now()}`
+      const optimisticTodo: Todo = {
+        id: optimisticId,
+        text,
+        createdAt: Date.now(),
+      }
+
+      queryClient.setQueryData<Array<Todo>>(['todos'], (old = []) => [
+        ...old,
+        optimisticTodo,
+      ])
+
+      return { previousTodos, optimisticId }
+    },
+    onError: (_err, _text, context) => {
+      setLastError(_err.message)
+      if (context?.previousTodos !== undefined) {
+        queryClient.setQueryData(['todos'], context.previousTodos)
+      } else if (context?.optimisticId) {
+        queryClient.setQueryData<Array<Todo>>(['todos'], (old = []) =>
+          old.filter((todo) => todo.id !== context.optimisticId),
+        )
+      }
+    },
+    onSettled: () => {
+      void queryClient.invalidateQueries({ queryKey: ['todos'] })
+    },
+  })
+
+  function handleSubmit(e: React.FormEvent) {
+    e.preventDefault()
+    const text = inputValue.trim()
+    if (!text) return
+    addTodoMutation.mutate(text)
+    setInputValue('')
+  }
+
+  return (
+    <div>
+      <p style={{ fontSize: '0.875rem', color: '#555', marginBottom: '1rem' }}>
+        <strong>Approach 2 — via cache manipulation:</strong>{' '}
+        <code>onMutate</code> snapshots the cache and writes the optimistic item
+        in. <code>onError</code> restores the snapshot on failure.
+      </p>
+
+      <form
+        onSubmit={handleSubmit}
+        style={{ display: 'flex', gap: 8, marginBottom: '1rem' }}
+      >
+        <input
+          value={inputValue}
+          onChange={(e) => setInputValue(e.target.value)}
+          placeholder="New todo…"
+          style={{ flex: 1, padding: '0.4rem 0.6rem', fontSize: '1rem' }}
+        />
+        <button type="submit" disabled={addTodoMutation.isPending}>
+          Add
+        </button>
+      </form>
+
+      {lastError && (
+        <p style={{ color: 'red', marginBottom: '0.5rem' }}>{lastError}</p>
+      )}
+
+      <ul style={{ listStyle: 'none', padding: 0 }}>
+        {todos.map((todo) => (
+          <li
+            key={todo.id}
+            style={{
+              padding: '0.4rem 0',
+              borderBottom: '1px solid #eee',
+              opacity: todo.id.startsWith('optimistic-') ? 0.5 : 1,
+            }}
+          >
+            {todo.text}
+            {todo.id.startsWith('optimistic-') && <em> (saving…)</em>}
+          </li>
+        ))}
+      </ul>
+    </div>
+  )
+}