JSFiddle - React, Tailwind, and code Playground

HTML

<select id="datepicker">
    <option selected="selected">Choose a date</option>
    <option>-------------</option>
    <option value="2/10/2011">2/10/2011</option>
    <option value="3/10/2011">3/10/2011</option>
    <option value="4/10/2011">4/10/2011</option>
</select>

<div id="dateDiv" style="display:none;">
    <!-- This div contains dates from database -->
    <ul id="dates">
        <li>2/10/2011</li>
        <li>4/10/2011</li>
    </ul>
</div>

JavaScript

$('#datepicker').change(function() {
    // Get the value of date field
    var dateValue = $('#datepicker').val();
    
    // Make and empty list
    var dateList = [];
    
    // Append all dates from <li> to dateList
    $("#dates li").each(function() {
        dateList.push($(this).text()); 
    });

    // Perform the match    
    if ($.inArray(dateValue, dateList  ) > -1) {
        alert("Date matched!!!");
    } else {
        alert("Sorry. Try again.");
    }
});