initialise the object angular2

  export class DashComponents
    implements OnInit {

dashtest: IDashPosttest[];


ngOnInit(): void
this works 
this.FirstName = "testing it all";
  <input type="text" class="form-control"  [value]="FirstName"  (input)="FirstName = $event.target.value"/>

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but this does not work
this.dashtest[0]FirstName = "testing it all"
 <input type="text" class="form-control"  #firstnameRef="ngModel" [(ngModel)]="dash.FirstName"   name="firstName"  required />;

export interface IDashPosttest {
    FirstName: string;
    LastName: string;
}

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Error receiving Error: Error in :0:0 caused by: Cannot read property '0' of undefined