interface IgxSummaryResult {
  key: string;
  label: string;
  summaryResult: any;
}

class CustomSummary extends IgxSummaryOperand {
  constructor() {
    super();
  }
  public operate(data: any[], allData: any[], fieldName: string, groupRecord: IGroupByRecord): IgxSummaryResult[] {
    const result = [];
    result.push({
      key: "test",
      label: "Test",
      summaryResult: IgxSummaryOperand.count(data)
    });
    return result;
  }
}
this.grid.getColumnByName('ColumnName').summaries = CustomSummary;

IgxSummaryOperand.count(dataSource);