How can i set focus on a newly (automatically) rendered dom element?

I have an input field that gets replaced automatically with a textarea and same content depending on the number of characters the user has entered:

<textarea v-if="myString.length > 20" v-model="myString"/>
<input type="text" v-if="myString.length <= 20" v-model="myString"/>

The problem i have is that the focus gets lost when a user enters the 21st character. And thus the user gets irritated because when he types the 22nd character it does not appear in the textarea (no focus). How can i set the focus on the newly rendered textarea then? Problem here is that it gets rendered automatically. Otherwise i could set a ref on the textarea and call focus().

Another issue is the removal of the 21st character and the switch-back from textarea to the input elment.

Answer

You could wrap the textarea/input in a component, and use its mounted hook to call its focus(), as seen in this component:

<!-- AutoFocusedInput.vue -->
<script setup>
import { ref, onMounted, computed, nextTick } from 'vue'

const input = ref()

onMounted(async () => {
  await nextTick()
  input.value.focus()
})

const props = defineProps({
  modelValue: String,
  textarea: Boolean,
})

const comp = computed(() => (props.textarea ? 'textarea' : 'input'))
</script>

<template>
  <component
    :is="comp"
    ref="input"
    v-bind="$attrs"
    :value="modelValue"
    @input="$emit('update:modelValue', $event.target.value)"
  />
</template>

<AutoFocusedInput textarea v-if="myString.length > 20" v-model="myString" />
<AutoFocusedInput v-else v-model="myString" />

demo

While this is technically possible, this UX is probably not ideal, and you should consider other designs that don’t require focusing input like this (as indicated by @kien_coi_1997).