JSFiddle - React, Tailwind, and code Playground

JavaScript

var start = new Date("2018-01-11");
  var finish = new Date("2018-12-31");
  var dayMilliseconds = 1000 * 60 * 60 * 24;
  var weekendDays = 0;
  while (start.getTime() <= finish.getTime()) {
    var day = start.getDay();
    if (day == 0) {
        weekendDays++;
    }
    start = new Date(+start + dayMilliseconds);
  }
  alert(weekendDays);