JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" id="datepicker" />
<input type="text" id="week" />

JavaScript

Date.prototype.getWeek = function () {
    var onejan = new Date(this.getFullYear(), 0, 1);
    return Math.ceil((((this - onejan) / 86400000) + onejan.getDay() + 1) / 7);
}

$('#datepicker').datepicker({
    onSelect: function (date) {
        var dat = new Date(date);
        var weekNo = dat.getWeek();
        $('#week').val(weekNo);

    }
});