// ...
 const obj = reactive({
      prop1: 'prop1',
      prop2: 'prop2'
    })
watch(
      () => obj.prop1,
      (prop1, prevProp1) => {
        console.log("#######");
        console.log("Lets watch:");
        console.group();
        console.log("obj.prop1: " + prop1);
        console.log("Old prop1: " + prevProp1);
        console.groupEnd();
        console.group();
        console.log("Prop2: " + obj.prop2);
        console.groupEnd();
        console.log("End watch.");
        console.log("#######");        
      }
    )
//...