an illustration (non-trivial example) of many newer JS class features

1-CalendarItem.js

// abstract class, not intended to be instantiated directly
class CalendarItem {
    static #UNSET = Symbol("unset")
    static #isUnset(v) {
        return v === this.#UNSET;
    }
    static {
        for (let [idx,msg] of [
            "ID is already set.",
            "ID is unset.",
            "Don't instantiate 'CalendarItem' directly.",
        ].entries()) {
            this[`ERROR_${(idx+1)*100}`] = msg;
        }
    }
    static isSameItem(item1,item2) {
        if (#ID in item1 && #ID in item2) {
            return item1.#ID === item2.#ID;
        }
        else {
            return false;
        }
    }

    #ID = CalendarItem.#UNSET
    #setID(id) {
        if (CalendarItem.#isUnset(this.#ID)) {
            this.#ID = id;
        }
        else {
            throw new Error(CalendarItem.ERROR_100);
        }
    }

    description = null
    startDateTime = null

    constructor() {
        // enforcing: abstract class, no direct instantiation
        if (new.target !== CalendarItem) {
            let id = Math.round(Math.random() * 1e9);
            this.#setID(id);
        }
        else {
            throw new Error(CalendarItem.ERROR_300);
        }
    }
    getID() {
        if (!CalendarItem.#isUnset(this.#ID)) {
            return this.#ID;
        }
        else {
            throw new Error(CalendarItem.ERROR_200);
        }
    }
    getDateTimeStr() {
        if (this.startDateTime instanceof Date) {
            return this.startDateTime.toUTCString();
        }
    }
    summary() {
        console.log(`(${
            this.getID()
        }) ${
            this.description
        } at ${
            this.getDateTimeStr()
        }`);
    }
}

2-Reminder.js

class Reminder extends CalendarItem {
    #complete = false;  // <-- no ASI, semicolon needed

    [Symbol.toStringTag] = "Reminder"
    constructor(description,startDateTime) {
        super();

        this.description = description;
        this.startDateTime = startDateTime;
    }
    isComplete() {
        return !!this.#complete;
    }
    markComplete() {
        this.#complete = true;
    }
    summary() {
        if (this.isComplete()) {
            console.log(`(${this.getID()}) Complete.`);
        }
        else {
            super.summary();
        }
    }
}

3-Meeting.js

class Meeting extends CalendarItem {
    #getEndDateTimeStr() {
        if (this.endDateTime instanceof Date) {
            return this.endDateTime.toUTCString();
        }
    }

    endDateTime = null;  // <-- no ASI, semicolon needed

    [Symbol.toStringTag] = "Meeting"
    constructor(description,startDateTime,endDateTime) {
        super();

        this.description = description;
        this.startDateTime = startDateTime;
        this.endDateTime = endDateTime;
    }
    getDateTimeStr() {
        return `${
            super.getDateTimeStr()
        } - ${
            this.#getEndDateTimeStr()
        }`;
    }
}

4.js

var callMyParents = new Reminder(
    "Call my parents to say hi",
    new Date("July 7, 2022 11:00:00 UTC")
);
callMyParents.toString();
// [object Reminder]
callMyParents.summary();
// (586380912) Call my parents to say hi at
// Thu, 07 Jul 2022 11:00:00 GMT
callMyParents.markComplete();
callMyParents.summary();
// (586380912) Complete.
callMyParents instanceof Reminder;
// true
callMyParents instanceof CalendarItem;
// true
callMyParents instanceof Meeting;
// false


var interview = new Meeting(
    "Job Interview: ABC Tech",
    new Date("June 23, 2022 08:30:00 UTC"),
    new Date("June 23, 2022 09:15:00 UTC")
);
interview.toString();
// [object Meeting]
interview.summary();
// (994337604) Job Interview: ABC Tech at Thu,
// 23 Jun 2022 08:30:00 GMT - Thu, 23 Jun 2022
// 09:15:00 GMT
interview instanceof Meeting;
// true
interview instanceof CalendarItem;
// true
interview instanceof Reminder;
// false


Reminder.isSameItem(callMyParents,callMyParents);
// true
Meeting.isSameItem(callMyParents,interview);
// false