JavaScript/DOM university assignment

assignment.js

// Add courses to the list (disallow adding same item twice)
// Remove courses from the list
// Limit courses by category
// Each course takes 5 credits, the list MUST have 6 courses
// Find desire course's category and suggest it on submit
// Course(or module) belongs to a Program
// Algebra belongs to math
// Linear belongs to math


// Up to 3 General
// Up to 4 Software
// Up to 4 Hardware
// Up to 2 Free

maximumCourses = 6;

let categories = {
    general : {
        title: 'General',
        courses: [
            'G1',
            'G2',
            'G3',
            'G4',
            'G5',
            'G6',
        ],
        limit: 3,
        selected: []
    },
    software : {
        title: 'Software',
        program: 'Computer Science',
        courses: [
            'S1',
            'S2',
            'S3',
            'S4',
            'S5',
            'S6',
        ],
        limit: 4,
        selected: []
    },
    hardware : {
        title: 'Hardware',
        program: 'Computer Engineering',
        courses: [
            'H1',
            '9H2',
            'H3',
            'H4',
            'H5',
            'H6',
        ],
        limit: 4,
        selected: []
    },
    free : {
        title: 'Free electives',
        courses: [
            'F1',
            'F2',
            'F3',
            'F4',
            'F5',
            'F6',
        ],
        limit: 2,
        selected: []
    },
};

function add(course) {
    let category = courseCategory(course);

    if (!category) {
        alert('Unknown category ' + category);
        return;
    }

    //Do not add again the same course
    if (categories[category].selected.indexOf(course) != -1) {
        alert('Selected course ' + course + ' is alread selected');
        return;
    }

    //Do not add based on category's limit
    if (categories[category].selected.length == categories[category].limit) {
        alert('You are allowed to add only ' + categories[category].limit + ' of ' + category);
        return;
    }

    //Prevent exceeding the maximum number of courses
    if (totalSelected() == maximumCourses) {
        alert('You have reached the maximum limit of courses.');
        return;
    }

    categories[category].selected.push(course);
    printSelected(course);
    printCreditsCounter();
}

function remove(course) {
    let category = courseCategory(course);
    let index = categories[category].selected.indexOf(course);
    if (index == -1) {
        alert('Delete error, course ' + course + ' not found');
        return;
    }

    categories[category].selected.splice(index, 1);

    //Print again the selected list
    printSelected();
    printCreditsCounter();
}

function printCreditsCounter() {
    document.getElementById('counter').innerHTML = totalSelected() * 5;
}

function totalSelected() {
    let total = 0;
    for (category in categories) {
        total += categories[category].selected.length;
    }
    return total;
}

function printSelected(course) {
    let selected = document.getElementById('selected');
    selected.innerHTML = "";
    for (category in categories) {
        for (index in categories[category].selected) {
            let element = document.createElement('li');
            let text = document.createTextNode(categories[category].selected[index]);
            element.appendChild(text);
            selected.appendChild(element);

        }
    }
    registerRemoveEvent();
}

function courseCategory(course) {
    for (category in categories) {
        if (categories[category].courses.indexOf(course) !== -1)
            return category;
    }

    return;
}

function loadLists() {
    for (category in categories) {
        let list = document.getElementById(category);
        //Clear list
        list.innerHTML = "";
        for (course in categories[category].courses) {
            let element = document.createElement('li');
            let text = document.createTextNode(categories[category].courses[course]);
            element.appendChild(text);
            list.appendChild(element);
        }
    }
    registerAddEvent();
}

function submitCourses() {
    let title = categories.software.title;
    let program = categories.software.program;
    if (categories.software.selected.length < categories.hardware.selected.length) {
        title = categories.hardware.title;
        program = categories.hardware.program;
    }

    // The rest of this code assumes you are not using a library.
    // It can be made less wordy if you use one.
    let form = document.createElement("form");
    form.setAttribute("method", 'POST');
    form.setAttribute("action", '/step3.php');


    let programField = document.createElement("input");
    programField.setAttribute("type", "hidden");
    programField.setAttribute("name", 'program');
    programField.setAttribute("value", program);
    form.appendChild(programField);

    let titleField = document.createElement("input");
    titleField.setAttribute("type", "hidden");
    titleField.setAttribute("name", 'title');
    titleField.setAttribute("value", title);
    form.appendChild(titleField);


    document.body.appendChild(form);

    form.submit();
}

function registerAddEvent() {
    let all = document.getElementById('all-courses').getElementsByTagName("li");
    for (let i = 0; i < all.length; i++) {
        all[i].addEventListener("click", (e) => {
            add(e.target.textContent);
        });
    }
}

function registerRemoveEvent() {
    let selected = document.getElementById('selected').getElementsByTagName("li");
    for (let i = 0; i < selected.length; i++) {
        selected[i].addEventListener("click", (e) => {
            remove(e.target.textContent);
        });
    }
}

i dont understand