如何解决无法读取属性错误？

Question

以下是在控制台中引发错误的代码块。该功能是在我的 Angular 应用程序的服务中实现的。

    lastEmployeeID()  //code block that's throwing error
    {
     let temp= this._http.get(this._employeesUrl).subscribe((employees:any) 
      =>  temp = employees,err => console.log(err));
     let last=temp[temp.length-1].id;
     return last;
    }    

这是错误：

错误类型错误：无法读取未定义的属性“id” EmployeesService.lastEmployeeID (employees.service.ts:43) 在 AddEmployeeFormComponent.onSubmit (addemployee-form.component.ts:19) 在 Object.eval [as handleEvent] (AddEmployeeFormComponent.html:3) 在 handleEvent (core.js:13542) 在 callWithDebugContext (core.js:15051) 在 Object.debugHandleEvent [as handleEvent] (core.js:14638) at dispatchEvent (core.js:9957) at eval (core.js:12296) at SafeSubscriber.schedulerFn [as _next] (core.js:4339) at SafeSubscriber.__tryOrUnsub (Subscriber.js:240)

Answer 1

我将尝试在这里解释问题：

lastEmployeeID() {
  // Here you're assigning the subscription to the get request to temp
  let temp= this._http.get(this._employeesUrl).subscribe(
    (employees:any) =>  temp = employees, // This will only happen when the get request completes
    err => console.log(err)
  );

  // By the time you get here, the get request will most likely not 
  // have finished and so "temp = employees" will not have been called. 
  // So you are trying to get temp[temp.length-1] where temp is not an 
  // array but rather a subscription.
  let last=temp[temp.length-1].id;
    return last;
  }    

这是您可以解决此问题的一种方法：

lastEmployeeID(): Observable<any> {
  return new Observable(observer => {
    this._http.get(this._employeesUrl).subscribe(employees => {
      observer.next(employees[employees.length-1].id)
    }, err => {
      console.log(err))
      observer.error(err)
    }
  })
}