js獲取日期周數

let date = new Date(2020, 1, 9);
let beginDate = new Date(date.getFullYear(), 0, 1);
let week = Math.ceil((parseInt((date - beginDate) / (24 * 60 * 60 * 1000)) + 1 + beginDate.getDay()) / 7);

//let dateCode = date.getFullYear().toString().substr(-2) + ('0' + week).substr(-2);