Angular2 - TypeError: Cannot read property 'Id' of undefined in (Typescript)

Question

我收到以下错误：

angular2.dev.js:23925 EXCEPTION: TypeError: Cannot read property 'Id' of null in [


{{ product.Id }}
 in ProductEditComponent@0:68]

抛出：

//Product-edit.component.ts:

import {Component } from 'angular2/core';
import { IProduct } from './product'
import { ProductService } from './product.service'
import { RouteParams } from 'angular2/router';
@Component({
  template:`<div class="wrapper wrapper-content animated fadeInRight ecommerce"> 
              {{ product.Id }}
            </div>`, 
})
export class ProductEditComponent{
    product: IProduct = null;
    errorMessage: string;
    constructor(private _routeParams: RouteParams, private _productService: ProductService){

    }

    ngOnInit(): void {
        this._productService.getProduct(this._routeParams.get('id'))
            .subscribe(
                product => this.product = product,
                error => this.errorMessage = <any>error);


    }

}

产品服务：

getProduct(id: string): Observable<IProduct> {
    return this._http.get(this._baseUrl + this._getProductUrl + '/' + id)
        .map((response: Response) => <IProduct>response.json())
        .do(data => console.log("All: " + JSON.stringify(data)))
        .catch(this.handleError);
}

来自服务器的响应：

{"Id":"34d4efcy6","ExternalId":null,"UserId":"testing","ProductProvider":null,"Title":"Flaska vin","Desc":null,"MinDeliveryTime":null,"MaxDeliveryTime":null,"FreightCost":null,"Brand":null}

我搞砸了什么？

Answer 1

在您的组件中，您将 product 初始化为 null，然后在您的模板中引用 product.Id。当 Angular 最初尝试绘制模板时会发生错误，在异步调用 returns 之前 - 此时 product 仍然为空，因此错误：Cannot read property 'Id' of null。

最直接的解决方案是使用 Elvis operator，Angular 正是为这种情况提供的。您可以通过在模板中将 {{ product.Id }} 替换为 {{ product?.Id }} 来使用它。

也就是说，您可能会运行使用这种方法来解决变更检测问题，并且通常情况下，使用以下方法会更好：

export class ProductEditComponent{
  product: Observable<IProduct>; //product is an Observable
  errorMessage: string;
  constructor(private _routeParams: RouteParams, private _productService: ProductService){
     //product is always defined because you instantiate it in the ctor
     this._productService.getProduct(this._routeParams.get('id'));
  }

然后您将在模板中使用 {{(product | async).Id }} 代替 {{product.Id}}，利用 AsyncPipe 让 angular 处理订阅和更新 UI 根据您的需要。

Angular2 - TypeError: Cannot read property 'Id' of undefined in (Typescript)

Angular2 - TypeError: Cannot read property 'Id' of undefined in (Typescript)

javascript

typescript

angular2-http

angular