js根据输入的日期获取当年的第几周

web开发中，我们经常遇到客户要显示第几周的等等麻烦的计算，那么输入当前日期如何快速查处是第几周呢？

let date = new Date('2021-02-28')

getWeekInYear(date)
function getWeekInYear(date) {
    console.log(date)
    //判断该星期是否跨年，如果跨年就是第一周
    let weekStartDate = getWeekStartByDate(date) //一周开始时间
    console.log(weekStartDate,'weekStartDate')
    let endDate = getDateFromDay(weekStartDate, 6) //一周结束时间
    console.log(endDate,'endDate')
    if (weekStartDate.getFullYear() != endDate.getFullYear()) {
        console.log(endDate.getFullYear() + '年第1周')
        let num = 1
        return num
    }
    let d1 = new Date(date)
    let d2 = new Date(date)
    d2.setMonth(0)
    d2.setDate(1)
    d2 = getWeekStartByDate(d2)
    console.log(d2,'d2')
    let rq = d1 - d2
    let days = Math.ceil(rq / (24 * 60 * 60 * 1000)) + 1
    let num = Math.ceil(days / 7)
    console.log(endDate.getFullYear() + '年第' + num + '周')
    return num
}
function getWeekStartByDate(date) {
    let day = date.getDay()
    return getDateFromDay(date, -day)
}
function getDateFromDay(dayDate, day) {
    let date = new Date()
    date.setTime(dayDate.getTime() + day * 24 * 60 * 60 * 1000)
    return date
}