every time a new object is pushed into array my loop in template repeats. how can I handle this in template?

Question

I want to print a list of cars whenever a new object is added to my existing array. the loop repeats and i print from start again. How can i avoid this ?

I tried using async pipe and also track by


// in the service
           getVehicles(){
        obj = { data: [{name: 'car 1'},{name: 'car 2'}] }
            return Observable.interval(2200).map(i=> obj.data.push({name: 'car 3'}));
        }


    // in the controller
    vehicles: Observable<Array<any>>
    ngOnInit() {
        this.vehicles = this._vehicleService.getVehicles().obj.data;
    }


// in template
<div *ngFor='let vehicle of vehicles | async'>
    {{vehicle.name}}
</div>

expected car 1 car 2 car 3 car 3

but it gives

car 1 car 2 car 1 car 2 car 3 car 1 car 2 car 3 car 3

Answer 1

I think this solution is very bad. I hope this is an exercise. The problem I think is this:

  vehicles: Observable<Array<any>>
    getVehicles(){
this.vehicles=[];
obj=null;
        obj = { data: [{name: 'car 1'},{name: 'car 2'}] }
            return Observable.interval(2200).map(i=> obj.data.push({name: 'car 3'}));
        }


    // in the controller

    ngOnInit() {
        this.vehicles = this._vehicleService.getVehicles().obj.data;
    }

Answer 2

Repeats in the view are probably caused by script errors (accessing some property of undefined) - isn't there anything in red in the console? A lot of red?

Anyway: to add cars, you can

getVehicles() {
   const obj = { data: [{name: 'car 1'},{name: 'car 2'}] };

   return interval(2200).pipe(
      map(() => {
         const data = [...obj.data, { name: 'car 3' }];
         return Object.assign(obj, { data });
      });
   );
}

To read them:

ngOnInit() {
   this.vehicles = this._vehicleService.getVehicles().pipe(
      map(({ data }) => data)
  );
}