TS2345:“字符串"类型的参数 |null' 不能分配给类型为 'string | 的参数网址树' [英] TS2345: Argument of type 'string | null' is not assignable to parameter of type 'string | UrlTree'

问题描述

这个问题的标题是 Angular CLI 编译器抛出的信息.

The title of this question is the message what the Angular CLI compiler throws.

我的 app.component.ts 中有这个构造函数:

I have this constructor in my app.component.ts:

    export class AppComponent {
      constructor(private userService: UserService, private auth: AuthService, router: Router) {
       auth.user$.subscribe(user => {
        if (user) {
         userService.save(user);

         let returnUrl = localStorage.getItem('returnUrl');
         router.navigateByUrl(returnUrl);
      }
    });
  }
}

在上面附加的代码部分中，我有下划线的returnURL"，如果我将光标放在上面，它会说:

In the above attached code part I have the 'returnURL' underlined and if I take the cursor above it says:

"'string 类型的参数 |null' 不可分配给类型为 'string | 的参数 |网址树'.类型 'null' 不能分配给类型 'string |UrlTree'."

"Argument of type 'string | null' is not assignable to parameter of type 'string | UrlTree'. Type 'null' is not assignable to type 'string | UrlTree'."

我有一个与 Firebase 相关的 user.service.ts:

I have a user.service.ts which is related to Firebase:

    import { Injectable } from '@angular/core';
    import { AngularFireDatabase } from 'angularfire2/database';
    import * as firebase from 'firebase';

    @Injectable()
    export class UserService {

        constructor(private db: AngularFireDatabase) { }
  
          save(user: firebase.User) {
          this.db.object('/users/' + user.uid).update({
            name: user.displayName,
            email: user.email
         });
       }
     }

有没有人知道这里编译器的问题是什么?我还附上了控制台消息.

Does anyone has an idea about what is the compiler's problem here? I also attached the console message.

推荐答案

该错误为您提供了所需的所有信息.

The error is giving you all of the information you need.

localStorage.getItem 返回字符串或 null，并且 router.navigateByUrl 需要 string 的参数code> 或 UrlTree.在调用 router.navigateByUrl 之前，您必须检查 localStorage.getItem 的返回值以确保它不为 null，因为您不能使用 null 调用它.

localStorage.getItem returns either a string or null, and router.navigateByUrl expects a parameter of either a string or a UrlTree. You'll have to check the return value of localStorage.getItem to make sure it's not null before calling router.navigateByUrl because you can't call it with null.

let returnUrl = localStorage.getItem('returnUrl');
if (returnUrl) { // i.e, not null and not empty string 
    // now returnUrl cannot be null, so it must be a string, which is valid to use in this call
    router.navigateByUrl(returnUrl);
}

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