• If the quantity of the current article is equal to 0, return message:

"The {articleName} is not available."

• If the guestName is not present in the guests array, return message:

"This guest is not invited."

;-)

    buyArticle(articleModel, articleName, guestName) {
        let findModel = this.listOfArticles.find((x) => { return x.articleModel === articleModel.toLowerCase() && x.articleName === articleName });
        let guest = this.guests.find((x) => { return x.guestName === guestName });

        if (!findModel) {
            throw new Error("This article is not found.")
        }

        if (findModel.quantity === 0) {
            // throw new Error(`The ${articleName} is not available.`) // no Error throwing!!!
            return `The ${articleName} is not available.`;
        }

        if (!guest) {
            // throw new Error("This guest is not invited."); // no Error throwing!!!
            return "This guest is not invited.";
        }

        let neededPoints = Number(this.possibleArticles[articleModel.toLowerCase()]);
        let userPoints = Number(guest.points);

        if (neededPoints <= userPoints) {
            guest.points -= neededPoints;
            findModel.quantity -= 1;
            guest.purchaseArticle += 1;
        } else {
            return "You need to more points to purchase the article.";
        }

        return `${guestName} successfully purchased the article worth ${neededPoints} points.`
    }