index.html

<html>
   <head>
      <title>VueJs Instance</title>
      <script src="https://unpkg.com/vue@3/dist/vue.global.js"></script>

   </head>
   <body>
      <div id = "computed_props">
         Kilometers : <input type = "text" v-model = "kilometers">
         Meters : <input type = "text" v-model = "meters">
      </div>
      <script>

        const { createApp } = Vue;
        
        const app = createApp({
            data(){
               kilometers : 0
               meters:0
            },
            methods: {
            },
            computed :{
            },
            watch : {
               kilometers:function(val) {
                  this.kilometers = val;
                  this.meters = val * 1000;
               },
               meters : function (val) {
                  this.kilometers = val/ 1000;
                  this.meters = val;
               }
            }
         }).mount("#computed_props");
      </script>
   </body>
</html>