JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" class="datepicker" name="anydate" id="anydate" />
<input type="text" id="weekstart" />
<input type="text" id="weekend" />

JavaScript

$(function() {
    $('#anydate').datepicker({
        showWeek:true,
        firstDay:0,
        dateFormat: 'dd-M-yy',
        onSelect: function() {
            var d = $(this).datepicker('getDate');
            var tstamp = $.datepicker.formatDate('@',d);
            var wd = d.getDay();
        
            var wkstart = new Date();
                alert(wkstart);
            wkstart.setDate(d.getDate() - wd);
            var wkend = new Date();
            wkend.setDate(d.getDate() + (7 - wd));
            $('#weekstart').val(wkstart.toDateString());
            $('#weekend').val(wkend.toDateString());
    }
    });
});