const emit = defineEmits<{ (e: 'update', value: string): void; }>();

const count = ref(0); const doubleCount = computed(() => count.value * 2);


### Composables (REQUIRED)

```typescript
// ✅ ALWAYS: Extract reusable logic into composables
// composables/useUser.ts
export function useUser(userId: Ref<string>) {
  const user = ref<User | null>(null);
  const loading = ref(true);
  
  watchEffect(async () => {
    loading.value = true;
    user.value = await fetchUser(userId.value);
    loading.value = false;
  });
  
  return { user, loading };
}

Reactive State (REQUIRED)

// ✅ Use ref for primitives
const count = ref(0);

// ✅ Use reactive for objects
const state = reactive({
  users: [] as User[],
  loading: false,
});

// ✅ Use computed for derived state
const activeUsers = computed(() => 
  state.users.filter(u => u.active)
);

Decision Tree

Need primitive state?      → Use ref()
Need object state?         → Use reactive()
Need derived value?        → Use computed()
Need side effect?          → Use watchEffect()
Need specific watch?       → Use watch()
Need reusable logic?       → Create composable

Code Examples

Component with v-model

<script setup lang="ts">
const model = defineModel<string>();
</script>

<template>
  <input :value="model" @input="model = $event.target.value" />
</template>

Async Component

const AsyncModal = defineAsyncComponent(() =>
  import('./components/Modal.vue')
);

Commands

npm create vue@latest myapp
npm run dev
npm run build
npm run test:unit

vue

Critical Patterns

Script Setup (REQUIRED)