class.js

class Period {
    constructor(year, month) {
        this.year = year;
        this.month = month;
    }

    next() {
        return this.month + 1 > 11
            ? new Period(this.year + 1, 0)
            : new Period(this.year, this.month + 1)
    }

    previous() {
        return this.month - 1 < 0
            ? new Period(this.year - 1, 11)
            : new Period(this.year, this.month - 1)
    }

    static parse(YYYY_MM) {
        const [year, month] = YYYY_MM.split('-').map(Number);
        return new Period(year, month - 1);
    }
}

// const Period = require('./Period')

// const p1 = new Period(2017, 10)
// const p2 = new Period(2018, 9)
// console.log(p1.next())

factory.js

const Period = (year, month) => ({
    year,
    month,

    parse(YYYY_MM) {
        const [year, month] = YYYY_MM.split('-').map(Number);
        return Period(year, month - 1);
    },

    next: () => month + 1 > 11
        ? Period(year + 1, 0)
        : Period(year, month + 1),

    previous: () => month - 1 < 0
        ? Period(year - 1, 11)
        : Period(year, month - 1),

});

// const Period = require('./Period')

// const p1 = Period(2017, 10)
// const p2 = Period(2018, 9)
// console.log(p1.next())

functional.js

const Period = (year, month) => ({ year, month });

const Period1 = {
    next: ({ year, month }) => month + 1 > 11
        ? Period(year + 1, 0)
        : Period(year, month + 1),

    previous: ({ year, month }) => month - 1 < 0
        ? Period(year - 1, 11)
        : Period(year, month - 1),

    parse(YYYY_MM) {
        const [year, month] = YYYY_MM.split('-').map(Number);
        return Period(year, month - 1);
    },
};

// const { nextPeriod: next } = require('./Period')

// const p1 = Period(2017, 10)
// const p2 = Period(2018, 9)
// console.log(nextPeriod(p1))