// useCounter.ts (Composable)
import { ref, computed } from 'vue';
export function useCounter(initialValue = 0) {
  const count = ref(initialValue);
  const doubled = computed(() => count.value * 2);
  const increment = () => count.value++;
  const decrement = () => count.value--;
  const reset = () => count.value = initialValue;
  return {
    count,
    doubled,
    increment,
    decrement,
    reset
  };
}
// Counter.vue
<template>
  <div class="counter">
    <p>Count: {{ count }}</p>
    <p>Doubled: {{ doubled }}</p>
    <button @click="increment">+</button>
    <button @click="decrement">-</button>
    <button @click="reset">Reset</button>
  </div>
</template>
<script setup lang="ts">
import { useCounter } from './useCounter';
const { count, doubled, increment, decrement, reset } = useCounter(0);
</script>
<style scoped>
.counter {
  padding: 20px;
  border: 1px solid #ccc;
}
</style>

// useFetch.ts
import { ref, computed, onMounted } from 'vue';
interface UseFetchOptions {
  immediate?: boolean;
}
export function useFetch<T>(
  url: string,
  options: UseFetchOptions = {}
) {
  const data = ref<T | null>(null);
  const loading = ref(false);
  const error = ref<Error | null>(null);
  const isLoading = computed(() => loading.value);
  const hasError = computed(() => error.value !== null);
  const fetch = async () => {
    loading.value = true;
    error.value = null;
    try {
      const response = await globalThis.fetch(url);
      if (!response.ok) throw new Error(`HTTP ${response.status}`);
      data.value = await response.json();
    } catch (e) {
      error.value = e instanceof Error ? e : new Error(String(e));
    } finally {
      loading.value = false;
    }
  };
  const refetch = () => fetch();
  if (options.immediate !== false) {
    onMounted(fetch);
  }
  return {
    data,
    loading: isLoading,
    error: hasError,
    fetch,
    refetch
  };
}
// UserList.vue
<template>
  <div>
    <button @click="refetch">Refresh</button>
    <p v-if="loading">Loading...</p>
    <p v-if="error" class="text-red-500">Error loading users</p>
    <ul v-else>
      <li v-for="user in data" :key="user.id">{{ user.name }}</li>
    </ul>
  </div>
</template>
<script setup lang="ts">
import { useFetch } from './useFetch';
interface User {
  id: number;
  name: string;
}
const { data, loading, error, refetch } = useFetch<User[]>('/api/users');
</script>

src/
├── components/
│   ├── common/
│   │   ├── Button.vue
│   │   ├── Card.vue
│   │   └── Modal.vue
│   ├── forms/
│   │   ├── FormInput.vue
│   │   └── FormSelect.vue
│   └── layouts/
│       ├── Header.vue
│       └── Sidebar.vue
├── composables/
│   ├── useCounter.ts
│   ├── useFetch.ts
│   └── useForm.ts
├── services/
│   ├── api.ts
│   └── auth.ts
├── stores/
│   ├── user.ts
│   └── auth.ts
├── types/
│   ├── models.ts
│   └── api.ts
├── App.vue
└── main.ts

// useForm.ts
import { ref, reactive } from 'vue';
interface UseFormOptions<T> {
  onSubmit: (data: T) => Promise<void>;
  initialValues: T;
}
export function useForm<T extends Record<string, any>>(
  options: UseFormOptions<T>
) {
  const formData = reactive<T>(options.initialValues);
  const errors = reactive<Record<string, string>>({});
  const isSubmitting = ref(false);
  const handleSubmit = async (e?: Event) => {
    e?.preventDefault();
    isSubmitting.value = true;
    try {
      await options.onSubmit(formData);
    } catch (error) {
      const err = error as any;
      if (err.fieldErrors) {
        Object.assign(errors, err.fieldErrors);
      }
    } finally {
      isSubmitting.value = false;
    }
  };
  const reset = () => {
    Object.assign(formData, options.initialValues);
    Object.keys(errors).forEach(key => delete errors[key]);
  };
  return {
    formData,
    errors,
    isSubmitting,
    handleSubmit,
    reset
  };
}
// LoginForm.vue
<template>
  <form @submit="handleSubmit">
    <input v-model="formData.email" type="email" />
    <span v-if="errors.email" class="error">{{ errors.email }}</span>
    <input v-model="formData.password" type="password" />
    <span v-if="errors.password" class="error">{{ errors.password }}</span>
    <button type="submit" :disabled="isSubmitting">Login</button>
  </form>
</template>
<script setup lang="ts">
import { useForm } from './useForm';
const { formData, errors, isSubmitting, handleSubmit } = useForm({
  initialValues: { email: '', password: '' },
  onSubmit: async (data) => {
    const response = await fetch('/api/login', {
      method: 'POST',
      body: JSON.stringify(data)
    });
    if (!response.ok) throw new Error('Login failed');
  }
});
</script>

// stores/user.ts
import { defineStore } from 'pinia';
import { ref, computed } from 'vue';
interface User {
  id: number;
  name: string;
  email: string;
}
export const useUserStore = defineStore('user', () => {
  const user = ref<User | null>(null);
  const isLoading = ref(false);
  const isLoggedIn = computed(() => user.value !== null);
  const fetchUser = async (id: number) => {
    isLoading.value = true;
    try {
      const response = await fetch(`/api/users/${id}`);
      user.value = await response.json();
    } finally {
      isLoading.value = false;
    }
  };
  const logout = () => {
    user.value = null;
  };
  return {
    user,
    isLoading,
    isLoggedIn,
    fetchUser,
    logout
  };
});
// Usage in component
import { useUserStore } from '@/stores/user';
export default {
  setup() {
    const userStore = useUserStore();
    userStore.fetchUser(1);
    return { userStore };
  }
};