离子HTTP数据返回问题

Question

我正在学习离子的所有东西。 Typescript，Angular和其他一些构成Ionic的技术对我来说都是新的。

运行以下代码：

import { Component } from '@angular/core';
import { IonicPage, NavController, NavParams } from 'ionic-angular';
import { CategoriesServiceProvider } from '../../providers/categories-service/categories-service';
import { CategoryPage } from '../category/category';

/**
 * Generated class for the CategoriesPage page.
 *
 * See https://ionicframework.com/docs/components/#navigation for more info on
 * Ionic pages and navigation.
 */

@IonicPage()
@Component({
  selector: 'page-categories',
  templateUrl: 'categories.html',
  providers: [CategoriesServiceProvider]
})
export class CategoriesPage {

  constructor(public navCtrl: NavController, public navParams: NavParams, private categories: CategoriesServiceProvider) {
  }

  quizCategories: any;

  getCategories(){
    this.categories.getCategories().then(question_categories => {
      if(question_categories.categories){
        this.quizCategories = question_categories.categories;
        console.log(this.quizCategories);
      }
    });
  }
}

我收到以下错误：

我添加了第28行，以检查 question_categories 是否正在返回数据，但同时检查和确定第29行上的分配会产生同样的问题。

我尝试读取的json数据如下：

{
    "categories": 
    [
        {
            "category": {
                "name": "Level 1",
                "count": "1",
                "tid": "3"
            }
        },
        {
            "category": {
                "name": "Level 2",
                "count": "2",
                "tid": "1"
            }
        }
    ]
}