Vue 3 Script Setup と defineModel 2026年版：モダン構文とインタビュー対策

<!-- UserCard.vue -->
<script setup lang="ts">
import { ref, computed } from 'vue'
import type { User } from '@/types'

// TypeScriptインターフェースを使用したProps
const props = defineProps<{
  user: User
  showEmail?: boolean
}>()

// リアクティブな状態
const isExpanded = ref(false)

// 算出プロパティ
const displayName = computed(() => {
  return `${props.user.firstName} ${props.user.lastName}`
})

// 型付きペイロードを持つイベントエミッター
const emit = defineEmits<{
  select: [userId: string]
  delete: [userId: string]
}>()

// メソッドは単純な関数として定義
function handleSelect() {
  emit('select', props.user.id)
}
</script>

<template>
  <div class="user-card" @click="handleSelect">
    <h3>{{ displayName }}</h3>
    <p v-if="showEmail">{{ user.email }}</p>
    <button @click.stop="isExpanded = !isExpanded">
      {{ isExpanded ? 'Collapse' : 'Expand' }}
    </button>
  </div>
</template>

<!-- SearchInput.vue -->
<script setup lang="ts">
import { watch } from 'vue'

// デフォルト値付きの分割代入 - 3.5以降でリアクティブ
const { query = '', placeholder = 'Search...' } = defineProps<{
  query?: string
  placeholder?: string
}>()

// watchは分割代入されたpropsでも動作
watch(
  () => query,
  (newQuery) => {
    console.log('Query changed:', newQuery)
  }
)
</script>

<template>
  <input
    :value="query"
    :placeholder="placeholder"
    @input="$emit('update:query', ($event.target as HTMLInputElement).value)"
  />
</template>

<!-- 従来の方法 - 冗長だが明示的 -->
<script setup lang="ts">
const props = defineProps<{ modelValue: string }>()
const emit = defineEmits<{ 'update:modelValue': [value: string] }>()

function updateValue(event: Event) {
  emit('update:modelValue', (event.target as HTMLInputElement).value)
}
</script>

<!-- TextInput.vue -->
<script setup lang="ts">
// defineModelはv-modelと同期するrefを作成
const model = defineModel<string>({ default: '' })
</script>

<template>
  <input
    v-model="model"
    type="text"
    class="text-input"
  />
</template>

<!-- ParentForm.vue -->
<script setup lang="ts">
import { ref } from 'vue'
import TextInput from './TextInput.vue'

const username = ref('')
</script>

<template>
  <TextInput v-model="username" />
  <p>Username: {{ username }}</p>
</template>

<!-- DateRangePicker.vue -->
<script setup lang="ts">
// 複数のv-modelバインディング用の名前付きモデル
const startDate = defineModel<Date>('start', { required: true })
const endDate = defineModel<Date>('end', { required: true })

// バリデーション用の算出プロパティ
const isValidRange = computed(() => {
  if (!startDate.value || !endDate.value) return true
  return startDate.value <= endDate.value
})
</script>

<template>
  <div class="date-range-picker">
    <input
      type="date"
      :value="startDate?.toISOString().split('T')[0]"
      @input="startDate = new Date(($event.target as HTMLInputElement).value)"
    />
    <span>to</span>
    <input
      type="date"
      :value="endDate?.toISOString().split('T')[0]"
      @input="endDate = new Date(($event.target as HTMLInputElement).value)"
    />
    <p v-if="!isValidRange" class="error">End date must be after start date</p>
  </div>
</template>

<DateRangePicker v-model:start="filterStart" v-model:end="filterEnd" />

<!-- CurrencyInput.vue -->
<script setup lang="ts">
// 2番目の戻り値でモディファイアにアクセス
const [model, modifiers] = defineModel<number>({
  default: 0,
  // カスタムモディファイア用の変換関数
  set(value) {
    // 'round'モディファイアが存在する場合は小数点以下2桁に丸める
    if (modifiers.round) {
      return Math.round(value * 100) / 100
    }
    return value
  }
})
</script>

<template>
  <div class="currency-input">
    <span>$</span>
    <input
      type="number"
      step="0.01"
      :value="model"
      @input="model = parseFloat(($event.target as HTMLInputElement).value) || 0"
    />
  </div>
</template>

<CurrencyInput v-model.round="price" />

<!-- GenericList.vue -->
<script setup lang="ts" generic="T extends { id: string }">
import { computed } from 'vue'

// ジェネリックコンポーネントのprops
const props = defineProps<{
  items: T[]
  selected?: T
}>()

const emit = defineEmits<{
  select: [item: T]
  remove: [id: string]
}>()

// スロットも型付け可能
defineSlots<{
  default: (props: { item: T; index: number }) => any
  empty: () => any
}>()

const hasItems = computed(() => props.items.length > 0)
</script>

<template>
  <ul v-if="hasItems">
    <li
      v-for="(item, index) in items"
      :key="item.id"
      :class="{ selected: selected?.id === item.id }"
      @click="emit('select', item)"
    >
      <slot :item="item" :index="index">
        {{ item.id }}
      </slot>
    </li>
  </ul>
  <div v-else>
    <slot name="empty">No items</slot>
  </div>
</template>

<!-- 移行前：Options API -->
<script lang="ts">
import { defineComponent, PropType } from 'vue'

export default defineComponent({
  props: {
    items: { type: Array as PropType<string[]>, required: true }
  },
  emits: ['select'],
  data() {
    return { searchTerm: '' }
  },
  computed: {
    filteredItems() {
      return this.items.filter(item =>
        item.toLowerCase().includes(this.searchTerm.toLowerCase())
      )
    }
  },
  methods: {
    selectItem(item: string) {
      this.$emit('select', item)
    }
  }
})
</script>

<!-- 移行後：Script Setup -->
<script setup lang="ts">
import { ref, computed } from 'vue'

const props = defineProps<{ items: string[] }>()
const emit = defineEmits<{ select: [item: string] }>()

const searchTerm = ref('')

const filteredItems = computed(() =>
  props.items.filter(item =>
    item.toLowerCase().includes(searchTerm.value.toLowerCase())
  )
)

function selectItem(item: string) {
  emit('select', item)
}
</script>