Nho Jotom Nho Jotom - 1 month ago 22
JSON Question

Getting an object array from an AngularJS 2 service

I am new to AngularJS 2 (and Javascript for that matter). I've written an AngularJS 2 service which returns an array of users. The data is retrieved from an HTTP call which returns the data in JSON format. When logging the JSON data returned from the HTTP call, I can see that this call is successful and the correct data is returned. I have a component which calls the service to get the users and an HTML page which displays the users. I cannot get the data from the service to the component. I suspect I am using the Observable incorrectly. Maybe I'm using subscribe incorrectly as well. If I comment out the getUsers call in the ngInit function and uncomment the getUsersMock call, everything works fine and I see the data displayed in the listbox in the HTML page. I'd like to convert the JSON data to an array or list of Users in the service, rather then returning JSON from the service and having the component convert it.

Data returned from HTTP call to get users:

[
{
"firstName": "Jane",
"lastName": "Doe"
},
{
"firstName": "John",
"lastName": "Doe"
}
]


user.ts

export class User {
firstName: string;
lastName: string;
}


user-service.ts

...
@Injectable
export class UserService {
private USERS: User[] = [
{
firstName: 'Jane',
lastName: 'Doe'
},
{
firstName: 'John',
lastName: 'Doe'
}
];

constructor (private http: Http) {}

getUsersMock(): User[] {
return this.USERS;
}

getUsers(): Observable<User[]> {
return Observable.create(observer => {
this.http.get('http://users.org').map(response => response.json();
})
}
...


user.component.ts

...
export class UserComponent implements OnInit {
users: User[] = {};

constructor(private userService: UserService) {}

ngOnInit(): void {
this.getUsers();
//this.getUsersMock();
}

getUsers(): void {
var userObservable = this.userService.getUsers();
this.userObservable.subscribe(users => { this.users = users });
}

getUsersMock(): void {
this.users = this.userService.getUsersMock();
}
}
...


user.component.html

...
<select disabled="disabled" name="Users" size="20">
<option *ngFor="let user of users">
{{user.firstName}}, {{user.lastName}}
</option>
</select>
...


!!! UPDATE !!!

I had been reading the "heroes" tutorial, but wasn't working for me so I went off and tried other things. I've re-implemented my code the way the heroes tutorial describes. However, when I log the value of this.users, it reports undefined.

Here is my revised user-service.ts

...
@Injectable
export class UserService {
private USERS: User[] = [
{
firstName: 'Jane',
lastName: 'Doe'
},
{
firstName: 'John',
lastName: 'Doe'
}
];

constructor (private http: Http) {}

getUsersMock(): User[] {
return this.USERS;
}

getUsers(): Promise<User[]> {
return this.http.get('http://users.org')
.toPromise()
.then(response => response.json().data as User[])
.catch(this.handleError);
}
...


Here is my revised user.component.ts

...
export class UserComponent implements OnInit {
users: User[] = {};

constructor(private userService: UserService) {}

ngOnInit(): void {
this.getUsers();
//this.getUsersMock();
}

getUsers(): void {
this.userService.getUsers()
.then(users => this.users = users);

console.log('this.users=' + this.users); // logs undefined
}

getUsersMock(): void {
this.users = this.userService.getUsersMock();
}
}
...

Answer

Take a look at your code :

 getUsers(): Observable<User[]> {
        return Observable.create(observer => {
            this.http.get('http://users.org').map(response => response.json();
        })
    }

and code from https://angular.io/docs/ts/latest/tutorial/toh-pt6.html (BTW. really good tutorial, you should check it out)

 getHeroes(): Promise<Hero[]> {
    return this.http.get(this.heroesUrl)
               .toPromise()
               .then(response => response.json().data as Hero[])
               .catch(this.handleError);
  }

The HttpService inside Angular2 already returns an observable, sou don't need to wrap another Observable around like you did here:

   return Observable.create(observer => {
        this.http.get('http://users.org').map(response => response.json()

Try to follow the guide in link that I provided. You should be just fine when you study it carefully.

---EDIT----

First of all WHERE you log the this.users variable? JavaScript isn't working that way. Your variable is undefined and it's fine, becuase of the code execution order!

Try to do it like this:

  getUsers(): void {
        this.userService.getUsers()
            .then(users => {
               this.users = users
               console.log('this.users=' + this.users);
            });


    }

See where the console.log(...) is!

Try to resign from toPromise() it's seems to be just for ppl with no RxJs background.

Catch another link: https://scotch.io/tutorials/angular-2-http-requests-with-observables Build your service once again with RxJs observables.