js 求一个月以前的时间

function getLastMonth(date) {
  const now = new Date(date)
	const year = now.getFullYear();
	const month = now.getMonth() + 1; // 0-11表示1-12月
	const day = now.getDate();
	const nowMonthDay = new Date(year, month, 0).getDate(); // 当前月的总天数
	let res = null
	if (month - 1 <= 0) {
		// 如果是1月，年数往前推一年
		res = year - 1 + "-" + 12 + "-" + day;
	} else {
		const lastMonthDay = new Date(year, parseInt(month) - 1, 0).getDate();
		if (lastMonthDay < day) {
			// 1个月前所在月的总天数小于现在的天日期
			if (day < nowMonthDay) {
				// 当前天日期小于当前月总天数
				res = year + "-" + (month - 1) + "-" + (lastMonthDay - (nowMonthDay - day));
			} else {
				res = year + "-" + (month - 1) + "-" + lastMonthDay;
			}
		} else {
			res = year + "-" + (month -1) + "-" + day;
		}
	}
	return res;
}

去除else后：

function getLastMonth(date) {
  const now = new Date(date)
	const year = now.getFullYear();
	const month = now.getMonth() + 1;
	const day = now.getDate();
	const nowMonthDay = new Date(year, month, 0).getDate(); // 当前月的总天数
	if (month - 1 <= 0) return year - 1 + "-" + 12 + "-" + day; // 如果是1月，年数往前推一年
	const lastMonthDay = new Date(year, parseInt(month) - 1, 0).getDate();
	if (lastMonthDay >= day) return year + "-" + (month -1) + "-" + day;
	if (day < nowMonthDay) return year + "-" + (month - 1) + "-" + (lastMonthDay - (nowMonthDay - day)); // 1个月前所在月的总天数小于现在的天日期
	return year + "-" + (month - 1) + "-" + lastMonthDay; // 当前天日期小于当前月总天数
}