App Vue + TypeScript problem...

App.vue

<script lang="ts">
import Vue from 'vue';
import Component from 'vue-class-component';

@Component({
  // Renders just “title: ”...
  template: '<div>title: {{ theTitle }}</div>'
})
export default class App extends Vue {
  private _title: string;

  constructor() {
    super();
    this._title = 'Original Title';
    console.info(this._title); // 'Original Title'.
  }

  get theTitle (): string {
    console.info(this._title); // undefined!!!!
    return this._title;
  }
};
</script>

Problem found.

Properties that start with _ or $ will not be proxied on the Vue instance because they may conflict with Vue’s internal properties and API methods. You will have to access them as vm.$data._property.

https://vuejs.org/v2/api/#Options-Data