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Render Function Patterns and Performance

.agents/skills/vue-best-practices/references/render-functions.md

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Render Function Patterns and Performance

Impact: MEDIUM - Render functions are powerful but opt out of template compiler optimizations. Use them intentionally and apply the key patterns below to keep output correct and performant.

Task List

  • Prefer templates; use render functions only when templates cannot express the logic
  • Always add stable keys when rendering lists with h()/JSX
  • Use withModifiers / withKeys for event modifiers
  • Implement v-model via modelValue + onUpdate:modelValue
  • Apply custom directives with withDirectives
  • Use functional components for stateless presentational UI

Prefer templates over render functions

BAD:

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

const count = ref(0)
const render = () => h('div', `Count: ${count.value}`)
</script>

GOOD:

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

const count = ref(0)
</script>

<template>
  <div>Count: {{ count }}</div>
</template>

Always add keys for list rendering

BAD:

javascript
import { h, ref } from 'vue'

export default {
  setup() {
    const items = ref([{ id: 1, name: 'Apple' }])

    return () => h('ul', items.value.map(item => h('li', item.name)))
  }
}

GOOD:

javascript
import { h, ref } from 'vue'

export default {
  setup() {
    const items = ref([{ id: 1, name: 'Apple' }])

    return () => h('ul', items.value.map(item => h('li', { key: item.id }, item.name)))
  }
}

Use withModifiers / withKeys for event modifiers

BAD:

javascript
import { h } from 'vue'

export default {
  setup() {
    const handleClick = (e) => {
      e.stopPropagation()
      e.preventDefault()
    }

    return () => h('button', { onClick: handleClick }, 'Click')
  }
}

GOOD:

javascript
import { h, withKeys, withModifiers } from 'vue'

export default {
  setup() {
    const handleClick = () => {}
    const handleEnter = () => {}

    return () => h('div', [
      h('button', {
        onClick: withModifiers(handleClick, ['stop', 'prevent'])
      }, 'Click'),
      h('input', {
        onKeyup: withKeys(handleEnter, ['enter'])
      })
    ])
  }
}

Implement v-model explicitly

BAD:

javascript
import { h, ref } from 'vue'

import CustomInput from './CustomInput.vue'

export default {
  setup() {
    const text = ref('')
    return () => h(CustomInput, { modelValue: text.value })
  }
}

GOOD:

javascript
import { h, ref } from 'vue'

import CustomInput from './CustomInput.vue'

export default {
  setup() {
    const text = ref('')
    return () => h(CustomInput, {
      'modelValue': text.value,
      'onUpdate:modelValue': (value) => { text.value = value }
    })
  }
}

Use withDirectives for custom directives

BAD:

javascript
import { h } from 'vue'

const vFocus = { mounted: el => el.focus() }

export default {
  setup() {
    return () => h('input', { 'v-focus': true })
  }
}

GOOD:

javascript
import { h, withDirectives } from 'vue'

const vFocus = { mounted: el => el.focus() }

export default {
  setup() {
    return () => withDirectives(h('input'), [[vFocus]])
  }
}

Prefer functional components for stateless UI

BAD:

javascript
import { h } from 'vue'

export default {
  setup() {
    return () => h('span', { class: 'badge' }, 'New')
  }
}

GOOD:

javascript
import { h } from 'vue'

function Badge(props, { slots }) {
  return h('span', { class: 'badge' }, slots.default?.())
}

Badge.props = ['variant']

export default Badge