Why can't I update its element value correctly within Array.forEach() loop in Angular 7? - arrays

Question

I'm testing Material Table(mat-table) on Angular 7, here's a weird issue I ran into.
Send a request to jsonplaceholder for fake data in users.service
export class UsersService {
API_BASE = 'https://jsonplaceholder.typicode.com/users';
constructor(private http: HttpClient) {}
getUsers(): Observable<object> {
const url = this.API_BASE;
return this.http.get(url);
}
}
Because jsonplaceholder only returns 10 rows of data, so I concatenate the data for a larger array, say, 30 rows for testing pagination feature with ease. Meanwhile, update the 'id' field with iterate index so the 'id's looks like 1,2,3...30, instead of 1,2,3...10,1,2,3...10,1,2,3...10, which is a result of concatenation, that's it, nothing special.
users.component:
ngOnInit(): void {
this.userService.getUsers().subscribe((users: UserData[]) => {
users = users.concat(users, users);
users.forEach((user, index) => (user.id = index +1));
console.log(users);
this.dataSource.data = users;
});
this.dataSource.paginator = this.paginator;
this.dataSource.sort = this.sort;
}
Although the table shows up beautifully, but the 'id's column looks weird, they are not 1,2,3...30 sequentially, instead, they are 21,22,23...30,21,22,23...30,21,22,23...30
I tried to print out the user.id inside the forEach loop, it's all good.
users.forEach((user, index) => {
user.id = index + 1;
console.log(user.id);
});
Where did I go wrong with this? Any clue? Thanks.
P.S, API used in the code: https://jsonplaceholder.typicode.com/users

Answer 1

even though you have 30 array elements after concatenating the array twice, you still only have 10 unique objects. the Object behind users[20] is the same as users[0], so you override the id of the already processed objects from index 10 to 29
you can fix this by creating a copy of each object. There are many ways too do this. a very simple way is serializing and deserializing using JSON.stringify and JSON.parse:
users.forEach(user => users.push(JSON.parse(JSON.stringify(user))));