calc-grade.user.js

// ==UserScript==
// @name           calc-grade
// @namespace      crabtw@gmail.com
// @description    calculate average grades
// @include        https://portal.yzu.edu.tw/Personal/Student/student_learn2.asp
// ==/UserScript==

var table = document.getElementsByClassName('table_1')[0].firstChild;
var iter = document.evaluate(
    'tr[position() > 4 and position() != last()]',
    table,
    null,
    XPathResult.UNORDERED_NODE_ITERATOR_TYPE,
    null
);

var tr = iter.iterateNext();
var totalCredits = 0;
var totalGrades = 0;

var totalPoints = 0;
var missedCredits = 0;

while(tr) {
    var td = tr.cells;
    var grade = parseInt(td[6].innerHTML);
    var credit = parseInt(td[5].innerHTML);

    if(grade >= 60 && grade <= 100) {
        totalCredits += credit
        totalGrades += grade * credit;
    } else {
        missedCredits += credit;
    }

    var point = Math.floor(grade / 10) - 4;
    if(point > 4) point = 4;
    totalPoints += credit * point;

    tr = iter.iterateNext();
}

var lastRow = document.createElement('tr');
var creditCell = document.createElement('td');
var gradeCell = document.createElement('td');
var averageCell = document.createElement('td');
var pointCell = document.createElement('td');

creditCell.innerHTML = totalCredits;
creditCell.align = 'center';
gradeCell.innerHTML = totalGrades;
gradeCell.align = 'center';
pointCell.innerHTML = totalPoints / (totalCredits + missedCredits);
pointCell.align = 'center';
averageCell.innerHTML = totalGrades / totalCredits;
averageCell.colSpan = 4;
averageCell.align = 'center';

lastRow.appendChild(averageCell);
lastRow.appendChild(pointCell);
lastRow.appendChild(creditCell);
lastRow.appendChild(gradeCell);

table.insertBefore(lastRow, table.lastChild);