javascript - Angular 2 : A property set during OnInit is undefined on the template

Question

我有这个组件:

export class CategoryDetailComponent implements OnInit{
  category: Category;
  categoryProducts: Product[];
  errorMessage: string;
  constructor(private _categoryService: CategoryService, private _productService: ProductService, private _routeParams: RouteParams ) {}

  ngOnInit() {
    this.getCategoryAndProducts();
  }
  getCategoryAndProducts() {
    let categoryName = this._routeParams.get('name');
    let categoryId = this.routeParams.get('id');
    var params = new URLSearchParams();
    params.set('category', categoryName);

    Observable.forkJoin(
      this._categoryService.getCategory(categoryId),
      this._productService.searchProducts(params)
    ).subscribe(
      data => {
      //this displays the expected category's name.
      console.log("category's name: "+ data[0].attributes.name)
      this.category = data[0];
      this.categoryProducts = data[1];
      }, error => this.errorMessage = <any>error
    )
  }
}

在组件的模板中我有这个:

<h1>{{category.attributes.name}}</h1>

当我导航到该组件时，出现错误:

TypeError: cannot read property 'attributes' of undefined

为什么模板上的 category 属性未定义，我该如何解决？

Answer 1

模板中的绑定(bind)在 ngOnInit() 之前求值。为防止 Angular 抛出错误，您可以使用

<h1>{{category?.attributes.name}}</h1>

Elvis 运算符会阻止 Angular 评估 .attributes... 除非 category 有值。