useOptimistic

import { useOptimistic, startTransition } from 'react';
import { updateName } from './actions.js';

export default function EditName({ name, action }) {
  const [optimisticName, setOptimisticName] = useOptimistic(name);

  async function submitAction(formData) {
    const newName = formData.get('name');
    setOptimisticName(newName);

    const updatedName = await updateName(newName);
    startTransition(() => {
      action(updatedName);
    })
  }

  return (
    <form action={submitAction}>
      <p>Your name is: {optimisticName}</p>
      <p>
        <label>Change it: </label>
        <input
          type="text"
          name="name"
          disabled={name !== optimisticName}
        />
      </p>
    </form>
  );
}

import { useOptimistic, startTransition } from 'react';

export default function Button({ action, children }) {
  const [isPending, setIsPending] = useOptimistic(false);

  return (
    <button
      disabled={isPending}
      onClick={() => {
        startTransition(async () => {
          setIsPending(true);
          await action();
        });
      }}
    >
      {isPending ? 'Submitting...' : children}
    </button>
  );
}

import { useState, useOptimistic, startTransition } from 'react';
import { toggleLike } from './actions.js';

export default function App() {
  const [isLiked, setIsLiked] = useState(false);
  const [optimisticIsLiked, setOptimisticIsLiked] = useOptimistic(isLiked);

  function handleClick() {
    startTransition(async () => {
      const newValue = !optimisticIsLiked
      console.log('⏳ setting optimistic state: ' + newValue);

      setOptimisticIsLiked(newValue);
      const updatedValue = await toggleLike(newValue);

      startTransition(() => {
        console.log('⏳ setting real state: ' + updatedValue );
        setIsLiked(updatedValue);
      });
    });
  }

  if (optimisticIsLiked !== isLiked) {
    console.log('✅ rendering optimistic state: ' + optimisticIsLiked);
  } else {
    console.log('✅ rendering real value: ' + optimisticIsLiked);
  }


  return (
    <button onClick={handleClick}>
      {optimisticIsLiked ? '❤️ Unlike' : '🤍 Like'}
    </button>
  );
}

import { useOptimistic, startTransition } from 'react';

export default function FollowButton({ user, followAction }) {
  const [optimisticState, updateOptimistic] = useOptimistic(
    { isFollowing: user.isFollowing, followerCount: user.followerCount },
    (current, isFollowing) => ({
      isFollowing,
      followerCount: current.followerCount + (isFollowing ? 1 : -1)
    })
  );

  function handleClick() {
    const newFollowState = !optimisticState.isFollowing;
    startTransition(async () => {
      updateOptimistic(newFollowState);
      await followAction(newFollowState);
    });
  }

  return (
    <div>
      <p><strong>{user.name}</strong></p>
      <p>{optimisticState.followerCount} followers</p>
      <button onClick={handleClick}>
        {optimisticState.isFollowing ? 'Unfollow' : 'Follow'}
      </button>
    </div>
  );
}

import { useOptimistic, startTransition } from 'react';

export default function TodoList({ todos, addTodoAction }) {
  const [optimisticTodos, addOptimisticTodo] = useOptimistic(
    todos,
    (currentTodos, newTodo) => [
      ...currentTodos,
      { id: newTodo.id, text: newTodo.text, pending: true }
    ]
  );

  function handleAddTodo(text) {
    const newTodo = { id: crypto.randomUUID(), text: text };
    startTransition(async () => {
      addOptimisticTodo(newTodo);
      await addTodoAction(newTodo);
    });
  }

  return (
    <div>
      <button onClick={() => handleAddTodo('New todo')}>Add Todo</button>
      <ul>
        {optimisticTodos.map(todo => (
          <li key={todo.id}>
            {todo.text} {todo.pending && "(Adding...)"}
          </li>
        ))}
      </ul>
    </div>
  );
}

import { useOptimistic, startTransition } from 'react';

export default function ShoppingCart({ cart, cartActions }) {
  const [optimisticCart, dispatch] = useOptimistic(
    cart,
    (currentCart, action) => {
      switch (action.type) {
        case 'add':
          const exists = currentCart.find(item => item.id === action.item.id);
          if (exists) {
            return currentCart.map(item =>
              item.id === action.item.id
                ? { ...item, quantity: item.quantity + 1, pending: true }
                : item
            );
          }
          return [...currentCart, { ...action.item, quantity: 1, pending: true }];
        case 'remove':
          return currentCart.filter(item => item.id !== action.id);
        case 'update_quantity':
          return currentCart.map(item =>
            item.id === action.id
              ? { ...item, quantity: action.quantity, pending: true }
              : item
          );
        default:
          return currentCart;
      }
    }
  );

  function handleAdd(item) {
    startTransition(async () => {
      dispatch({ type: 'add', item });
      await cartActions.add(item);
    });
  }

  function handleRemove(id) {
    startTransition(async () => {
      dispatch({ type: 'remove', id });
      await cartActions.remove(id);
    });
  }

  function handleUpdateQuantity(id, quantity) {
    startTransition(async () => {
      dispatch({ type: 'update_quantity', id, quantity });
      await cartActions.updateQuantity(id, quantity);
    });
  }

  const total = optimisticCart.reduce(
    (sum, item) => sum + item.price * item.quantity,
    0
  );

  return (
    <div>
      <h2>Shopping Cart</h2>
      <div style={{ marginBottom: 16 }}>
        <button onClick={() => handleAdd({
          id: 1, name: 'T-Shirt', price: 25
        })}>
          Add T-Shirt ($25)
        </button>{' '}
        <button onClick={() => handleAdd({
          id: 2, name: 'Mug', price: 15
        })}>
          Add Mug ($15)
        </button>
      </div>
      {optimisticCart.length === 0 ? (
        <p>Your cart is empty</p>
      ) : (
        <ul>
          {optimisticCart.map(item => (
            <li key={item.id}>
              {item.name} - ${item.price} ×
              {item.quantity}
              {' '}= ${item.price * item.quantity}
              <button
                onClick={() => handleRemove(item.id)}
                style={{ marginLeft: 8 }}
              >
                Remove
              </button>
              {item.pending && ' ...'}
            </li>
          ))}
        </ul>
      )}
      <p><strong>Total: ${total}</strong></p>
    </div>
  );
}

import { useState, useOptimistic, startTransition } from 'react';

export default function ItemList({ items, deleteAction }) {
  const [error, setError] = useState(null);
  const [optimisticItems, removeItem] = useOptimistic(
    items,
    (currentItems, idToRemove) =>
      currentItems.map(item =>
        item.id === idToRemove
          ? { ...item, deleting: true }
          : item
      )
  );

  function handleDelete(id) {
    setError(null);
    startTransition(async () => {
      removeItem(id);
      try {
        await deleteAction(id);
      } catch (e) {
        setError(e.message);
      }
    });
  }

  return (
    <div>
      <h2>Your Items</h2>
      <ul>
        {optimisticItems.map(item => (
          <li
            key={item.id}
            style={{
              opacity: item.deleting ? 0.5 : 1,
              textDecoration: item.deleting ? 'line-through' : 'none',
              transition: 'opacity 0.2s'
            }}
          >
            {item.name}
            <button
              onClick={() => handleDelete(item.id)}
              disabled={item.deleting}
              style={{ marginLeft: 8 }}
            >
              {item.deleting ? 'Deleting...' : 'Delete'}
            </button>
          </li>
        ))}
      </ul>
      {error && (
        <p style={{ color: 'red', padding: 8, background: '#fee' }}>
          {error}
        </p>
      )}
    </div>
  );
}