agent-skills/skills/vue-best-practices/reference/prop-validation-before-instance.md

title, impact, impactDescription, type, tags

title	impact	impactDescription	type	tags
Props Are Validated Before Component Instance Creation	MEDIUM	Instance properties like data and computed are unavailable in prop default/validator functions	gotcha	vue3 props validation lifecycle default-values

Props Are Validated Before Component Instance Creation

Impact: MEDIUM - Prop validation and default value functions execute before the component instance is created. This means this, data, computed, injections, and other instance properties are not available inside default or validator functions.

This timing catches developers who expect to use component state in prop validation logic.

Task Checklist

• Never reference this or instance properties in prop default/validator functions
• Use factory functions for object/array defaults that only use the function parameters
• For validation depending on other props, use the second props parameter in validators
• Move complex validation logic to watchers or lifecycle hooks if instance access is needed

Incorrect:

<script>
export default {
  data() {
    return {
      validOptions: ['a', 'b', 'c'],
      defaultMessage: 'Hello'
    }
  },
  props: {
    option: {
      type: String,
      // WRONG: 'this' is undefined during prop validation
      validator(value) {
        return this.validOptions.includes(value)  // TypeError!
      }
    },
    message: {
      type: String,
      // WRONG: Cannot access data properties
      default() {
        return this.defaultMessage  // TypeError!
      }
    },
    config: {
      type: Object,
      // WRONG: Cannot access computed properties
      default() {
        return this.computedDefaults  // TypeError!
      }
    }
  }
}
</script>

Correct:

<script>
// Define validation data outside the component
const VALID_OPTIONS = ['a', 'b', 'c']
const DEFAULT_MESSAGE = 'Hello'

export default {
  props: {
    option: {
      type: String,
      // CORRECT: Use external constants
      validator(value) {
        return VALID_OPTIONS.includes(value)
      }
    },
    message: {
      type: String,
      // CORRECT: Use external constant or inline value
      default: DEFAULT_MESSAGE
    },
    config: {
      type: Object,
      // CORRECT: Factory function with no instance dependencies
      default() {
        return { theme: 'light', size: 'medium' }
      }
    }
  }
}
</script>

Using the props Parameter in Validators

Validators receive the full props object as the second parameter for cross-prop validation:

<script setup>
const props = defineProps({
  min: {
    type: Number,
    default: 0
  },
  max: {
    type: Number,
    default: 100
  },
  value: {
    type: Number,
    required: true,
    // CORRECT: Access other props via second parameter
    validator(value, props) {
      return value >= props.min && value <= props.max
    }
  }
})
</script>

Using rawProps in Default Functions

Default factory functions receive rawProps for accessing other prop values:

<script setup>
defineProps({
  size: {
    type: String,
    default: 'medium'
  },
  padding: {
    type: Number,
    // Access other prop values via rawProps parameter
    default(rawProps) {
      // Use size to determine default padding
      return rawProps.size === 'large' ? 20 : 10
    }
  }
})
</script>

Post-Mount Validation Pattern

For validation that needs instance access, use lifecycle hooks:

<script setup>
import { onMounted, inject, warn } from 'vue'

const props = defineProps({
  theme: String
})

const availableThemes = inject('availableThemes', [])

// Validation that needs injected values
onMounted(() => {
  if (props.theme && !availableThemes.includes(props.theme)) {
    console.warn(`Invalid theme "${props.theme}". Available: ${availableThemes.join(', ')}`)
  }
})
</script>

Reference

• Vue.js Props - Prop Validation