Angular::error TS2532:对象可能是 'undefined'

Question

我正在做 Angular Tour-hero 项目(将 Hero 替换为“用户”)。当我将 Hero(User) 和 Hero-detail(User-detail) 分开并尝试访问详细信息时，它没有显示，更新功能也不起作用。它显示此错误:

error TS2532: Object is possibly 'undefined'.

6 <input id="user-name" [(ngModel)]="user.name"placeholder="name">

user 对象，我认为，给出了问题。但是，当我尝试完全按照本教程添加 MessageService 和所有内容时，它起作用了。但是当我删除所有这些时，它给出了这个错误。预先感谢您的帮助。

用户详细信息.component.html:

<div>
  <h2>{{user.name | uppercase}} Details</h2>
  <div><span>id: </span>{{user.id}}</div>
  <div>
        <label for="user-name">User name: </label>
        <input id="user-name" [(ngModel)]="user.name" placeholder="name">
    </div>
    <button (click)="goBack()">Back</button>
    <button (click)="save()">Save</button>
</div>

user-detail.component.ts:

import { Component, OnInit } from '@angular/core';
import { User } from 'src/app/model/user';
import { UserService } from 'src/app/services/user/user.service';
import { Location } from '@angular/common';
import { ActivatedRoute } from '@angular/router';


@Component({
  selector: 'app-user-detail',
  templateUrl: './user-detail.component.html',
  styleUrls: ['./user-detail.component.scss']
})
export class UserDetailComponent implements OnInit {
  user?: User

  constructor(
    private userService: UserService,
    private location: Location,
    private route: ActivatedRoute
  ) { }

  ngOnInit(): void {
    this.getUser();
  }

  getUser(): void {
    const id =
     parseInt(this.route.snapshot.paramMap.get('id')!, 1);
    this.userService.getUser(id)
      .subscribe(user => this.user = user);
  }

  goBack(): void {
    this.location.back();
  }

  save():void {
    if(this.user){
      this.userService.updateUser(this.user)
        .subscribe(() => this.goBack())
    }
  }

}

用户服务.ts:

import { Injectable } from '@angular/core';
import { User } from 'src/app/model/user';
import { HttpClient, HttpHeaders } from '@angular/common/http';
import { Observable, of } from 'rxjs';
import { catchError } from 'rxjs/operators';

@Injectable({
  providedIn: 'root'
})
export class UserService {
  
  private usersUrl = 'api/users/';

  httpOptions = {
    headers: new HttpHeaders({ 'Content-Type': 'application/json'})
  };

  getUsers(): Observable<User[]> {
    return this.http.get<User[]>(this.usersUrl).pipe(
      catchError(this.handleError<User[]>('getUsers',[]))
    );
  }

  /** GET hero by id. Will 404 if id not found */
  getUser(id: number): Observable<User> {
    const url = `${this.usersUrl}/${id}`;
    return this.http.get<User>(url).pipe(
      catchError(this.handleError<User>(`getUser id=${id}`))
    );
  }

  updateUser(user: User): Observable<any> {
    return this.http.put(this.usersUrl, user, this.httpOptions)
    .pipe(
        catchError(this.handleError<User[]>('getUsers',[]))
    );
  }

  addUser(user: User): Observable<User>{
    return this.http.post<User>(this.usersUrl, user, this.httpOptions)
      .pipe(
        catchError(this.handleError<User>('addUser'))
      )
  }

  deleteUser(id: number): Observable<User>{
    const url = `${this.usersUrl}/${id}`;

    return this.http.delete<User>(url, this.httpOptions).pipe(
      catchError(this.handleError<User>('deleteUser'))
    )
  }

  constructor(private http: HttpClient) { }

  private handleError<T>(operation = 'operation', result?:T){
    return (error: any): Observable<T> => {
      console.error(error);
      return of(result as T);
    }
  }
}

Answer 1

我相信这个错误是由于 typescript 2.1 或更高版本 引起的，您的代码非常完美!没有任何问题，但未初始化的变量减速方式可能会导致此错误，

现在如果你知道用户值或者你想用任何默认值初始化那么请像下面这样在构造函数中初始化你的用户变量

user: User;

constructor() {
  this.user = {
    id: 0,
    name: ''
  };
}

另一个解决方案是使用'Definite Assignment Assertion' 告诉 typescript 这个变量将在运行时有一个值，如下所示

组件.ts

user!: User;

还可以像下面这样对 component.html 文件进行更改

<div>
  <h2>{{user?.name! | uppercase}} Details</h2>
  <div><span>id: </span>{{user?.id!}}</div>
  <div>
    <label for="user-name">User name: </label>
    <input id="user-name" [(ngModel)]="user?.name!" placeholder="name">
  </div>
  <button (click)="goBack()">Back</button>
  <button (click)="save()">Save</button>
</div>