watch: {
    items: {
        handler: function(revised, original) {
            for (let i = 0; i < revised.length; i++) {
                if (isChanged(original, revised[i])) {
                    // update
                }
            }
        }
    }
  },
  methods: {
      isChanged(original, item) {
          // code here to check if item is changed
      }
    }
  }

[Vue warn]: Error in callback for watcher "items": "ReferenceError: isChanged is not defined"