How to keep the original index of an array when concatenating?

Question

JavaScript

There is a table with a list of students, there is also a form for searching by last name. The problem is that after the search, in the table the serial number of the found student is 1, and not the original number before the search.

The code below is used to search for students by last name.

searchStudent(lastname) {
                if (lastname !== "") {
                    this.dropStudents();
                    const group = this.state.currentGroup;
                    let arr = [];
                    for (let i = 0; i < group.length; i++) {
                        if (lastname.toLowerCase() === group[i].lastname.toLowerCase()) {
                            arr = arr.concat([group[i]]);
                        }
                    }
                    this.setState({ currentStudents: arr });
                } else if (lastname === "") {
                    this.dropStudents();
                    this.setState({ currentStudents: this.state.currentGroup });
                }
            }
            dropStudents() {
                this.setState({ currentStudents: [] });
            }

The result is displayed in the table:

if (this.props.currentGroup.length !== 0) {
                    const currentGroup = this.props.currentGroup;
                    return (
                        <section className="json-teachers">
                            <table><tbody>
                            {
                                currentGroup.map((student,index) => {
                                    return (
                                        <tr key={index} className="line">
                                            <td>{index + 1}</td>
                                            <td>{student.name}</td>
                                            <td>{student.lastname}</td>
                                            <td>{student.status}</td>
                                        </tr>
                                    );
                                })
                            }
                            </tbody></table>
                        </section>
                );
                } else { return null;}