SOURCE

function getWeekOfYear(){
  var today = new Date();
  var firstDay = new Date(today.getFullYear(),0, 1);
  var dayOfWeek = firstDay.getDay(); 
  var spendDay= 1;
  if (dayOfWeek !=0) {
    spendDay=7-dayOfWeek+1;
  }
  firstDay = new Date(today.getFullYear(),0, 1+spendDay);
  var d =Math.ceil((today.valueOf()- firstDay.valueOf())/ 86400000);
  var result =Math.ceil(d/7);
  console.log(result+1);
  return result+1;
}

<button onclick="getWeekOfYear()">aa</button>