@jsbits/add-months/index.js

Adds or subtracts N months to any JavaScript Date, local or UTC.

"use strict";
/**
 * Increment or decrement the given date by `count` months.
 *
 * @param {Date} date Local date
 * @param {number} count Months to add or subtract
 * @returns {Date} New local date
 * @private
 */
var addMonthsLoc = function (date, count) {
    // Get the day of this date, we just need to take care of days >28
    var day = date.getDate();
    // Set the month and change the day if it's above 28 to avoid overflow
    date.setMonth(date.getMonth() + count, day > 28 ? 28 : day);
    // Restore the day if necessary, preserving the month and we are done.
    if (day > 28) {
        var month = date.getMonth();
        /*
        "If there is an overflow in the day, the date is adjusted to the last
          valid day of the expected month."
        */
        date.setDate(day);
        if (date.getMonth() !== month) {
            date.setDate(0);
        }
    }
    return date;
};
/**
 * Like the `addMonthsLoc` function, but using UTC methods.
 *
 * @param {Date} date UTC date
 * @param {number} count Months to add or subtract
 * @returns {Date} New UTC date
 * @private
 */
var addMonthsUTC = function (date, count) {
    var day = date.getUTCDate();
    date.setUTCMonth(date.getUTCMonth() + count, day > 28 ? 28 : day);
    if (day > 28) {
        var month = date.getUTCMonth();
        date.setUTCDate(day);
        if (date.getUTCMonth() !== month) {
            date.setUTCDate(0);
        }
    }
    return date;
};
/** Shortcut */
var _toString = Object.prototype.toString;
/**
 * Convert a number or Date object to a Date, for other types, returns
 * an invalid date (i.e. NaN).
 *
 * @param {*} src Date or number
 * @return {Date} Parsed date
 * @private
 */
var toDate = function (src) {
    var type = _toString.call(src);
    return new Date(type === '[object Date]' || type === '[object Number]' ? +src : NaN);
};
/**
 * Returns a date occurring `count` months after `startdate` or, if `count` is
 * negative, the date occurring `count` months before `startdate`.
 *
 * - If `startdate` is not a Date or number that can be converted to a
 *    valid date, returns a new Date instance with an invalid date.
 *
 * - If `count` is evaluated as zero, returns a new Date instance with the
 *    the same value as `startdate`.
 *
 * - If there is an overflow in the day, the date is adjusted to the last
 *    valid day of the expected month.
 *
 * The third parameter is optional and indicates if the date is UTC. It is
 * necessary to differentiate UTC dates from locals and avoid errors due to the
 * [Daylight Saving Time](https://en.wikipedia.org/wiki/Daylight_saving_time)
 * (DST).
 *
 * This function does not change the original date.
 *
 * @param {Date|number} startdate A value parseable as a JavaScript Date
 * @param {number} count Number of months to add or subtract
 * @param {boolean} [asUTC=false] If `true`, handle the date as UTC
 * @returns {Date} A new, adjusted Date instance.
 * @since 1.0.0
 */
var addMonths = function _addMonths(startdate, count, asUTC) {
    // Create a new `Date` instance.
    // Wrong types return NaN dates without throwing exceptions.
    var date = toDate(startdate);
    // Corce `count` with 'ToInt32'
    count |= 0;
    // Filter out zero count and invalid dates.
    if (!count || isNaN(date)) {
        return date;
    }
    // Return the result with the UTC methods if required by the flags
    return asUTC ? addMonthsUTC(date, count) : addMonthsLoc(date, count);
};
module.exports = addMonths;
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