输入type=“日期”，显示周数

<input type="date" id="dateInput">
<button onclick="getWeekNumber()">显示周数</button>

function getWeekNumber() {
  var dateInput = document.getElementById("dateInput").value;
  var date = new Date(dateInput);
  var weekNumber = getISOWeek(date);
  alert("该日期对应的周数为：" + weekNumber);
}

function getISOWeek(date) {
  var target = new Date(date.valueOf());
  var dayNumber = (date.getDay() + 6) % 7;
  target.setDate(target.getDate() - dayNumber + 3);
  var firstThursday = target.valueOf();
  target.setMonth(0, 1);
  if (target.getDay() !== 4) {
    target.setMonth(0, 1 + ((4 - target.getDay()) + 7) % 7);
  }
  return 1 + Math.ceil((firstThursday - target) / 604800000);
}