Vue 3: watch

App.vue

<template>
  <div>
    <input v-model="name" />
    <p>{{ msg }}</p>
  </div>
</template>

<script>
import { reactive, toRefs, computed, watch  } from "vue";

export default {
  setup () {

    const data = reactive({
      name: 'Guillaume',
      msg: computed(() => { 
        return `Hello ${data.name}`
      })
    });

    watch(() => data.msg, (newValue, oldValue) => {
      console.log(newValue)
      console.log(oldValue)
    });

    return {
      ...toRefs(data)
    };
  }
}
</script>