todoapp.component.html

<input type="text" [(ngModel)]="todo" class="form-control" />
<hr />
<button class="btn btn-primary btn-block" (click)="aFireService.add(todo); todo = ''">AÑADIR TODO</button>
<hr />

<div *ngIf="(todos | async); let data">
  <table class="table" *ngIf="data.length > 0">
    <thead>
      <tr>
        <th>ID</th>
        <th>TODO</th>
        <th>DELETE</th>
      </tr>
    </thead>
    <tbody>
      <tr *ngFor="let todo of data">
        <td>{{ todo.id }}</td>
        <td>{{ todo.todo }}</td>
        <td>
          <button class="btn btn-danger" (click)="aFireService.remove(todo.id)">DELETE</button>
        </td>
      </tr>
    </tbody>
  </table>

  <div *ngIf="data.length === 0" class="alert alert-danger text-center">NO HAY TODOS</div>

</div>