질문

<!--
    Student 속성
    1) 값   : name, kor, eng, mat
    2) 함수
        (1) 평균 반환하는 getAverage
        (2) 학점 반환하는 getGrade 
-->
<div id="box5">
    <table border="1">
        <thead>
            <tr>
                <td>이름</td>
                <td>국어</td>
                <td>영어</td>
                <td>수학</td>
                <td>평균</td>
                <td>학점</td>
            </tr>
        </thead>
        <tbody id="student_info">

        </tbody>
    </table>
</div>
<script>
    // 문제) 임의의 Student 객체를 3개 생성해서 student_info에 표시할 것

    function Student(name, kor, eng, mat) {
        this.name = name;
        this.kor = kor;
        this.eng = eng;
        this.mat = mat;
        getAverage(this.name,this.kor,this.mat);
        getGrade((this.name,this.kor,this.mat)/3);

    }
    function getAverage(kor, eng, mat) {
        return (kor+eng+mat)/3;
    }
    function getGrade(average) {
        grade = '';
        switch(average%10){
            case 10 :
            case 9: grade = 'A'; break;
            case 8: grade = 'B'; break;
            case 7: grade = 'C'; break;
            case 6: grade = 'D'; break;
            default : grade = 'F'; break;  
            return grade;
        }
    }

    var student1 = new Student('정은지', 34,65, 23);
    var student2 = new Student('김채은', 34,65, 23);
    var student3 = new Student('권채은', 34,65, 23);
    students = [student1,student2,student3];
    console.log(student1, typeof student1);
    
    var stulist='';
    for(let i=0; i<students.length; i++) {
        stulist += '<tr>';
            stulist += '<td>'+students[i].name+'</td>';
            stulist += '<td>'+students[i].kor+'</td>';
            stulist += '<td>'+students[i].eng+'</td>';
            stulist += '<td>'+students[i].mat+'</td>';
            stulist += '<td>'+students[i].getAverage+'</td>';
            stulist += '<td>'+students[i].getGrade+'</td>';
        stulist += '</tr>';
    }

    document.getElementById('student_info').innerHTML=stulist;

</script>

undefined 가 나오는 이유는 .. ?