get index of row in angular material table v5

Using indexOf is an enormous waste of resources. The right way is initializing a variable with index’s value, like this:

<ng-container matColumnDef="position">
  <th mat-header-cell *matHeaderCellDef mat-sort-header> Num. </th>
  <td mat-cell *matCellDef="let element; let i = index">{{i + 1}}</td>
</ng-container>

https://material.angular.io/components/table/examples

UPDATE:

If you are using pagination the index can be calculated this way:

<ng-container matColumnDef="position">
  <th mat-header-cell *matHeaderCellDef mat-sort-header> Num. </th>
  <td mat-cell *matCellDef="let i = index">
  {{this.paginator.pageIndex == 0 ? i + 1 : 1 + i + this.paginator.pageIndex * this.paginator.pageSize}}
  </td>
</ng-container>