js获取当年第一天

function getFirstDayOfYear(date = new Date()) {
    const year = date.getFullYear();
    return new Date(year, 0, 1); // 注意：月份是从0开始的，所以0代表1月
}

const firstDayOfYear = getFirstDayOfYear();
console.log(firstDayOfYear); // 输出当年第一天的Date对象

function getFirstDayOfYearISO(date = new Date()) {
    const year = date.getFullYear();
    return `${year}-01-01`;
}

const firstDayOfYearISO = getFirstDayOfYearISO();
console.log(firstDayOfYearISO); // 输出格式为"YYYY-MM-DD"的字符串

function getFirstDayOfYearUTC(date = new Date()) {
    const year = date.getUTCFullYear();
    return new Date(Date.UTC(year, 0, 1));
}

const firstDayOfYearUTC = getFirstDayOfYearUTC();
console.log(firstDayOfYearUTC); // 输出UTC时间下的当年第一天的Date对象