Vue

HTML

<div id="app">
  <button @click="test1 = { ...test1, asdf: 'blah'}">Test with property change</button>
</div>

Vue

new Vue({
  el: "#app",
  data() {
    return {
      test1: {},
    }
  },
  computed: {
		test() {
      return this.test1
    }
  },
  watch: {
    test: {
      handler(val, oldVal) {
        console.log(oldVal, val);
      },
      deep: true
    }
  }
})