Type Error while hot reloading

[SimonSiefke]

Version

2.6.10

Reproduction link

https://github.com/SimonSiefke/vue-hot-reloading-issue

Steps to reproduce

Inside the script section inside App.vue, do the following edits:

1. remove the return inside data

export default {
    data() { }
};

2. return an empty object

export default {
  data() {
    return {};
  }
};

3. return the original

export default {
  data() {
    return {
      count: 2
    };
  }
};

What is expected?

After doing the edits, the App looks normal since it is the same code as before.

What is actually happening?

Hot reloading encounters multiple TypeErrors inside updateComponent from Vue and the App cannot recover from its state and doesn't update automatically anymore, it just shows a blank page.

Also, the command line shows DONE Compiled successfully in 97ms which suggests that everything is fine.

When the page is reloaded everything works again as expected.

---

Additional Information:

I have included images inside the minimal reproduction sample that might be useful.

[SimonSiefke]

The TypeErrors are happening in the following places:

1. vue/src/core/instance/proxy.js

   Line 59 in 78c3ce0

   (typeof key === 'string' && key.charAt(0) === '_' && !(key in target.$data))

target.$data is undefined, therefore we get TypeError: right-hand side of 'in' should be an object, got undefined

2. vue/src/core/instance/proxy.js

   Line 61 in 78c3ce0

   if (key in target.$data) warnReservedPrefix(target, key)

basically the same as 1.

3. vue/src/core/instance/lifecycle.js

   Line 119 in d9b27a9

   if (vm._data.__ob__) {

   
   vm.data is undefined, therefore we get TypeError: "vm._data is undefined"