无法读取未定义 angular2 ngif 的属性

问题描述

我现在可以在视图中获取对象，但是我无法运行 if 语句.根据之前的回答，这就是我引入对象的方式.

I am now able to get the object in the view however I cannot run an if statement. Per previous answer this is how I am bringing in the object.

public getPosts$(category, limit) {
  return this.cartService.getPosts(category, limit).map(response => {
    return response && response.data && response.data.children;
  };
}

ngOnInit() {
  this.getPosts$(this.category, this.limit).subscribe(cart => {
    this.cart = cart;
  }
}

我正在尝试运行，但无法获得属性蔬菜.

I am trying to run but get cannot get property vegetable.

<h2 *ngIf="cart">{{cart.vegetable}}</h2>
<h2 *ngIf="cart.vegetable == 'carrot' ">{{cart.vegetable}}</h2>

错误是

无法读取未定义的属性vegtable"

Cannot read property 'vegtable' of undefined

推荐答案

cart 对象为空，直到服务 getPosts$ 返回(回调).因此，代码 *ngIf="cart.vegetable ... 等于 *ngIf="null.vegetable ... 直到发生这种情况.这就是正在发生的事情.

The cart object is null until the service getPosts$ returns (callback). Therefore, the code *ngIf="cart.vegetable ... is equal to *ngIf="null.vegetable ... until that happens. That is what is happening.

您可以做的是放置一个带有 *ngIf="cart" 的 DOM 元素，其中包含另一个 *ngIf.例如:

What you could do is put a DOM element with *ngIf="cart" containing the other *ngIf. For example:

<div *ngIf="cart">
    <h2 *ngIf="cart.vegetable == 'carrot' ">{{cart.vegetable}}</h2>
</div>

*正如在下一个答案中所说，一个很好的选择(和良好的做法)如下:

* As it is said in the next answer, a good alternative (and good practice) is the following:

<h2 *ngIf="cart?.vegetable == 'carrot' ">{{cart.vegetable}}</h2>

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