defineProps Cannot Access Variables from script setup

Impact: MEDIUM - Code inside the defineProps() argument cannot access other variables declared in <script setup>. The entire expression is moved to an outer function scope when compiled, making local variables inaccessible.

This commonly surprises developers trying to use imported constants or computed validation logic.

Task Checklist

• [ ] Define validation constants outside <script setup> or in a separate file
• [ ] Import constants before using them in defineProps
• [ ] Use external type definitions for TypeScript props
• [ ] For dynamic validation, use watchers instead of prop validators

Incorrect:

<script setup>
import { ref } from 'vue'

// These are in <script setup> scope
const VALID_SIZES = ['sm', 'md', 'lg']
const maxLength = ref(100)

defineProps({
  size: {
    type: String,
    // WRONG: VALID_SIZES is not accessible here
    validator: (v) => VALID_SIZES.includes(v)  // ReferenceError!
  },
  name: {
    type: String,
    // WRONG: Cannot access refs
    validator: (v) => v.length <= maxLength.value  // ReferenceError!
  }
})
</script>

Correct:

<script>
// Define constants in regular <script> block (module scope)
export const VALID_SIZES = ['sm', 'md', 'lg']
export const MAX_LENGTH = 100
</script>

<script setup>
// Now accessible in defineProps
defineProps({
  size: {
    type: String,
    validator: (v) => VALID_SIZES.includes(v)  // Works!
  },
  name: {
    type: String,
    validator: (v) => v.length <= MAX_LENGTH  // Works!
  }
})
</script>

Pattern: Import from External File

// validation.js
export const VALID_SIZES = ['sm', 'md', 'lg']
export const VALID_COLORS = ['red', 'blue', 'green']
export const sizeValidator = (v) => VALID_SIZES.includes(v)

<script setup>
import { VALID_SIZES, VALID_COLORS, sizeValidator } from './validation'

// Imported values ARE accessible
defineProps({
  size: {
    type: String,
    validator: sizeValidator
  },
  color: {
    type: String,
    validator: (v) => VALID_COLORS.includes(v)
  }
})
</script>

Pattern: Dual Script Blocks

<script>
// Regular script for module-level declarations
const options = {
  themes: ['light', 'dark', 'system'],
  defaults: {
    theme: 'light',
    size: 'md'
  }
}
</script>

<script setup>
// options is accessible here
const props = defineProps({
  theme: {
    type: String,
    default: options.defaults.theme,
    validator: (v) => options.themes.includes(v)
  }
})
</script>

TypeScript: External Type Definitions

// types.ts
export interface UserProps {
  name: string
  email: string
  age?: number
}

<script setup lang="ts">
import type { UserProps } from './types'

// Type imports work fine
const props = defineProps<UserProps>()
</script>

Why This Happens

Vue's compiler transforms <script setup> code. The defineProps() call is extracted and moved to component options at compile time, before the setup function runs:

// Your code:
const MY_CONST = 'value'
defineProps({ prop: { default: MY_CONST } })

// Compiled (simplified):
export default {
  props: { prop: { default: MY_CONST } },  // MY_CONST doesn't exist here!
  setup() {
    const MY_CONST = 'value'  // Defined too late
  }
}

Reference

• Vue.js Script Setup - defineProps