typescript Angular 2 TypeError：无法读取未定义的属性“toUpperCase”
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Angular 2 TypeError: Cannot read property 'toUpperCase' of undefined
提问by Chebbi Ala Eddine
I am developing a medical website , i want show alert in page of medicament.component.HTML when the quantity of medicaments is < 50 . I did this :
我正在开发一个医疗网站，当药物数量小于 50 时，我想在 medicament.component.HTML 的页面中显示警报。我这样做了：
<span *ngFor="let medicament of medicaments" >
<span *ngIf="{{medicament.quantity}} < 50">
<div class="callout callout-danger lead">
<h4>Alert!</h4>
<p>Vous devez charger le stock de medicaments {{medicament.nom}}
de type {{medicament.type}} </p>
</div>
</span></span>
But i don't know why it doesn't work , the error is :
但我不知道为什么它不起作用，错误是：
> Unhandled Promise rejection: Template parse errors:
TypeError: Cannot read property 'toUpperCase' of undefined ("<span
*ngIf="medicaments">
<span *ngFor="let medicament of medicaments" >
<span [ERROR ->]*ngIf="{{medicament.quantity}} < 50">
<div class="callout callout-danger lead">
"): MedicamentComponent@26:18
Can't bind to '*ngIf' since it isn't a known property of 'span'.
("<span *ngIf="medicaments">
<span *ngFor="let medicament of medicaments" >
<span [ERROR ->]*ngIf="{{medicament.quantity}} < 50">
<div class="callout callout-danger lead">
"): MedicamentComponent@26:18
回答by Sasxa
You shouldn't use interpolation {{ }}in ngIf; it expects an expression:
你不应该{{ }}在 ngIf 中使用插值；它需要一个表达式：
<span *ngIf="medicament.quantity < 50">
回答by Merx
ngIf is already in the angular context, which means, that you must remove the brackets {{}}
ngIf 已经在 angular 上下文中，这意味着您必须删除方括号 {{}}
回答by Jai
Here in your logs you can see the point of error:
在您的日志中，您可以看到错误点：
<span [ERROR ->]*ngIf="{{medicament.quantity}} < 50">
and it is because you are using an expression on a property where you shouldn't. You just need to use property you are comparing:
这是因为您在不应该使用的属性上使用了表达式。您只需要使用要比较的属性：
<span *ngIf="medicament.quantity < 50">
