# addMonth.mjs

    import ot from 'dayjs'
    import ismonth from './ismonth.mjs'
    import isint from './isint.mjs'

    /**
     * 輸入年月並增加/減少月份數量
     *
     * Unit Test: {@link https://github.com/yuda-lyu/wsemi/blob/master/test/addMonth.test.mjs Github}
     * @memberOf wsemi
     * @param {String} g 輸入月時間字串
     * @param {Integer} n 輸入要增加的月份數量整數，可為正負值
     * @returns {String} 回傳新的月時間字串
     * @example
     *
     * console.log(addMonth('2019-01', 2))
     * // => '2019-03'
     *
     * console.log(addMonth('2018-12', -1))
     * // => '2018-11'
     *
     */
    function addMonth(g, n) {

        //check
        if (!ismonth(g)) {
            return ''
        }
        if (!isint(n)) {
            return ''
        }

        let m = ot(g, 'YYYY-MM')
        let r = m.add(n, 'months').format('YYYY-MM')

        return r
    }

    export default addMonth