Angular 2-TypeError：无法设置未定义的属性“ userId”

Question

I created a model and in a method when I want to give values the the model's properties, I get this error: TypeError: Cannot set property 'userId' of undefined

Here is my Model:

export class ApplicationUser {
    constructor(
        public id: string,
        public userId: string
    ) { }

}

Here is the method:

public alreadyExists(sub: string) {
    let applicationUser: ApplicationUser;

    this.applicationUserService.getApplicationUser(sub)
                                    .subscribe(
                                        appUser => applicationUser = appUser,
                                        error => this.errorMessage = <any>error
                                    );

    if (applicationUser == null) {
        applicationUser.userId = sub;

        this.applicationUserService.postApplicationUser(applicationUser)
                                       .subscribe(
                                           error => this.errorMessage = <any>error
                                       );
    }

    localStorage.setItem('userId', sub);
}

The error appears after this line: applicationUser.userId = sub;

What do I have to change/add to don't get this error message?

Thank You!

Answer 1

There are at least three issues with your code.

First you disregard the asynchronous nature of your code, second you try to write to a property of an object that is null and third you pass the error callback as the success callback to your post.

I don't know what 'sub' is but you seem to use it as userId so I do as well. Here is a replacement suggestion that does what you are trying to do. Instead of chaining the logic in promises I use the async/await construct.

public async alreadyExists(sub: string) {
  try {
    let applicationUser: ApplicationUser = 
      await this.applicationUserService.getApplicationUser(sub).toPromise();

    if (applicationUser === null) {
      applicationUser = new ApplicationUser(sub);

      await this.applicationUserService.postApplicationUser(applicationUser)
       .toPromise();
    }
    localStorage.setItem('userId', sub);
  } catch (error) {
    this.errorMessage = error;
  }
}

I hope this helps

Answer 2

let applicationUser: ApplicationUser;

The above line will not create the actual object instance here, its just a reference. Error says applicationUser is undefined

You can try doing

let applicationUser: ApplicationUser = new ApplicationUser ('', '');

Alternatively (not tested)

let applicationUser: ApplicationUser
 applicationUser = Object.create(ApplicationUser.prototype);
 this.constructor.apply(applicationUser, ['', '']);