angular2：错误：TypeError：无法读取未定义的属性“ ...” [英] angular2: Error: TypeError: Cannot read property '...' of undefined

问题描述

我已附上我的angular2代码段的小栓。我想从JSON打印一个字段，但由于最初我的Object为null，并且无法通过Promise进行填充，因此无法打印。

I have attached the plunker of my angular2 code piece. I want to print a field from my JSON but unable to print that as initially my Object is null and it is being populated via a Promise.

这是我的组件文件

import {Component, NgModule, OnInit} from '@angular/core'
import {BrowserModule} from '@angular/platform-browser'

class MyData {
  xyz : MySubData;
}

class MySubData {
  name : string;
} 
@Component({
  selector: 'my-app',
  template: `
    <div>
      <h2>Hello {{name}}</h2>

      {{abc.xyz.name}}
    </div>
  `,
})
export class App implements OnInit {
  abc : MyData = null;
  constructor() {
    this.name = 'Angular2'
  }

  ngOnInit() {
    setTimeout(() => {
      this.abc = new MyData();
      this.abc.xyz = new MySubData();
      this.abc.xyz.name = "Binita";
    }, 2000);
  }
}

@NgModule({
  imports: [ BrowserModule ],
  declarations: [ App ],
  bootstrap: [ App ]
})
export class AppModule {}

当我删除行 {{abc.xyz.name}} 在我的模板中运行正常。

When I am removing the line {{abc.xyz.name}} from my template it is running fine.

我已经使用了set在代码中超时，因为我正在从Promise中获取数据（即异步调用）。

I have use set time out in my code because I am getting my data from Promise (i.e asynchronous call).

最初我可以理解为 abc 为 null ，我的代码无法找到abc.xyz.name。但是我不想检查任何条件。因为我在JSON中有几个属性，所以如果条件满足，则不可能为每个属性写。

I can understand initially as abc is null, my code is unable to find abc.xyz.name. But I don't want to put any if condition to check. Because I have several property inside a JSON and it is not possible for each property to write if condition.

angularjs 1的早期版本，如果abc为null，那么它将自动替换它与空白字符串。我想在angular2中做同样的事情。请提出建议。

Earlier in angularjs 1 if abc is null then it would automatically replace it with blank string. I want to do the same thing in angular2. Please suggest.

下面是the车手

https://plnkr.co/edit/u1NqNF0penz7OS55QmoU?p=preview

推荐答案

这是因为在模板渲染时未定义 abc 。您可以使用安全的导航运算符（？）来保护模板，直到HTTP调用完成：

That's because abc is undefined at the moment of the template rendering. You can use safe navigation operator (?) to "protect" template until HTTP call is completed:

{{abc?.xyz?.name}}

您可以阅读有关安全导航操作员的详细信息，请此处。

You can read more about safe navigation operator here.

更新：

不能在数组中使用安全的导航运算符，您必须利用 NgIf 指令来克服此问题：

Safe navigation operator can't be used in arrays, you will have to take advantage of NgIf directive to overcome this problem:

<div *ngIf="arr && arr.length > 0">
    {{arr[0].name}}
</div>

了解有关 NgIf 指令此处。

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