how to get the latest row index in ag grid cell code example

Example: How to add new row to a particular index of a ag grid using angular 7

addRow(){
  // Assuming newRow is an object, such as {"slNo": this.index,"id":3, "rank":1};
  rowData.splice(targetIndex, 0, newRow);
  this.gridApi.setRowData(rowData); // optional
}