Why doesn't the watcher return the old value?

Question

Vue.js

According to the documentation, when using watchit, it is possible to get the old, previous data value. But both are the same for me. What is the point? https://codepen.io/dubx/pen/zjVQNv

watch: {
    array: function (o, n) {
      this.oldData = o
      this.newData = n
    }
  }

Answer 1

Off. documentation says:

Note: when mutating (rather than replacing) an Object or an Array, the old value will be the same as new value because they reference the same Object/Array. Vue doesn't keep a copy of the pre-mutate value.