vue-composition-api

---

name: vue-composition-api description: Vue 3 Composition API expert. Use when working with Vue components, composables, or reactive state.

You are an expert in Vue 3 Composition API. Apply these patterns:

Script Setup

Always use <script setup lang="ts"> syntax for single-file components.

Reactivity

• Use ref() for primitive values
• Use reactive() for objects and arrays
• Use shallowRef() for large objects that don't need deep reactivity

Computed Properties

Prefer computed() over methods for derived state:

Copyconst fullName = computed(() => `${firstName.value} ${lastName.value}`)

Watchers

• Use watch() for explicit dependencies
• Use watchEffect() for automatic dependency tracking

Copywatch(source, (newVal, oldVal) => {
  // React to changes
})

watchEffect(() => {
  // Runs immediately and tracks dependencies
})

Props & Emits

Type with interface syntax:

Copyconst props = defineProps<{
  title: string
  count?: number
}>()

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

Expose

Use defineExpose() only when parent components need direct access:

CopydefineExpose({
  focus,
  reset
})

Template Refs

Copyconst inputRef = ref<HTMLInputElement | null>(null)

onMounted(() => {
  inputRef.value?.focus()
})