LibraryCollection

class LibraryCollection{
    constructor(capacity){
        this.capacity = capacity;
        this.books = [];
    }

    addBook(bookName, bookAuthor){
        if(this.books.length >= this.capacity){
            throw new Error("Not enough space in the collection.");
        }

        let bookObj = {
            bookName,
            bookAuthor,
            payed: false,
        };

        this.books.push(bookObj);
        return `The ${bookName}, with an author ${bookAuthor}, collect.`;
    }

    payBook(bookName){
        let bookIndex = this.books.findIndex(x => x.bookName == bookName);

        if(bookIndex == -1){
            throw new Error(`${bookName} is not in the collection.`);
        }

        if(this.books[bookIndex].payed == true){
            throw new Error(`${bookName} has already been paid.`);
        }

        this.books[bookIndex].payed = true;
        return `${bookName} has been successfully paid.`;
    }

    removeBook(bookName){
        let bookIndex = this.books.findIndex(x => x.bookName == bookName);
        if(bookIndex == -1){
            throw new Error("The book, you're looking for, is not found.");
        }
        if(this.books[bookIndex].payed == false){
            throw new Error(`${bookName} need to be paid before removing from the collection.`);
        }

        this.books.splice(bookIndex,1);
        return `${bookName} remove from the collection.`;
    }

    getStatistics(bookAuthor){
        let output = [];

        if(typeof bookAuthor == 'undefined'){
            let emptySlots = this.capacity - this.books.length;
            output.push(`The book collection has ${emptySlots} empty spots left.`);
            this.books = this.books.sort((a,b) => {
                return a.bookName.localeCompare(b.bookName);
            }).forEach(x => {
                if(x.payed == true){
                    output.push(`${x.bookName} == ${x.bookAuthor} - Has Paid.`);
                }else{
                    output.push(`${x.bookName} == ${x.bookAuthor} - Not Paid.`);
                }
            })

            return output.join('\n');
        }else{
            let bookAuthorIndex = this.books.findIndex(x => x.bookAuthor == bookAuthor);

            if(bookAuthorIndex == -1){
                throw new Error(`${bookAuthor} is not in the collection.`)
            }

            if(this.books[bookAuthorIndex].payed){
                    return `${this.books[bookAuthorIndex].bookName} == ${this.books[bookAuthorIndex].bookAuthor} - Has Paid.`;
                }else{
                    return `${this.books[bookAuthorIndex].bookName} == ${this.books[bookAuthorIndex].bookAuthor} - Not Paid.`;
                }
        }

    }
}