I would like to know why i am getting the following error:

 error  'msg_' is assigned a value but never used 
 

despite that the variable msg_ is used in the following context as shown in the below posted code:

<HelloWorld :msg="msg_"/>

App.vue:

<template>
  <img alt="Vue logo" src="./assets/logo.png">
  <HelloWorld :msg="msg_"/>
</template>

<script>
import HelloWorld from './components/HelloWorld.vue'
import { ref } from 'vue'

export default {
  name: 'App',
  components: {
    HelloWorld
  },
  setup(props) {
    
    var msg_ = ref({msg:"msg from parent"})
  
    return {
      props
    }
  }
}
</script>

>Solution :

You have to return msg_ from the setup() function.

setup(props) {
  var msg_ = ref({msg:"msg from parent"})
  
  return {
    props,
    msg_,
  }
}

Also it better to use const and let instead of var.