displayFn(id) {
 return this.getValue(id)
}
getValue(id) {
 /**return some string
}

<mat-autocomplete #autoOutlet="matAutocomplete" [displayWith]="displayFn">
  <mat-option *ngFor="let option of outletFilterOptions | async [value]="option.outletId">
   {{ option.outletName }}
  </mat-option>
</mat-autocomplete>