Angular2 - TypeError:无法读取(Typescript)中未定义的属性"Id"

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);


    }

}

ProductService:

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);
}

服务器响应:

{"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

在组件中,您将初始化product为null,然后product.Id在模板中引用.当Angular 在异步调用返回之前尝试绘制模板时,会发生错误- 此时product仍然为null,因此错误:Cannot read property 'Id' of null.

最直接的解决方案是使用Elvis运算符,Angular提供了这样的情况.您{{ product.Id }}可以{{ product?.Id }}在模板中替换它来使用它.

也就是说,使用这种方法可能会遇到更改检测问题,一般来说,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{{(product | async).Id }},您可以使用代替{{product.Id}}模板,利用AsyncPipe角度来处理订阅和更新UI的脏工作.