JSFiddle - React, Tailwind, and code Playground

HTML

<html>
<body>

<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/qunit/1.11.0/qunit.min.css" />
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.0/jquery.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/qunit/1.11.0/qunit.min.js"></script>

<p>Type in a date in the format MMDDYYY.
    <br/>
    <input type="text" class="date" />
</p>
<div id="qunit"></div>
<div id="qunit-fixture"></div>
</body>
</html>

JavaScript

$(function(){
        $(".date").keyup(function(e){
                var currentValue = $(e.target).val();
                var newValue = autocompleteMMDDYYYYDateFormat( currentValue );
                if( newValue != currentValue ){
                        $(e.target).val( newValue );
                }
        });
});
/**
* This function helps to autocomplete the date format MMDDYYY
* Converts M to 0M and MMD to MM0D. Ex. `1/` to `01/`, `01/1/` to `01/01/`
* Adds slash for MM and MMDD Ex. `01` to `01/`, `01/02` to `01/02/`
* Converts YY to YYYY. Ex. `01/01/01` to `01/01/2001`
*
* @param {String} str
* @return {String}
*/
var autocompleteMMDDYYYYDateFormat = function (str) {
        str = str.trim();
        var matches, year,
                looksLike_MM_slash_DD = /^(\d\d\/)?\d\d$/,
                looksLike_MM_slash_D_slash = /^(\d\d\/)?(\d\/)$/,
                looksLike_MM_slash_DD_slash_DD = /^(\d\d\/\d\d\/)(\d\d)$/;

        if( looksLike_MM_slash_DD.test(str) ){
                str += "/";
        }else if( looksLike_MM_slash_D_slash.test(str) ){
                str = str.replace( looksLike_MM_slash_D_slash, "$10$2");
        }else if( looksLike_MM_slash_DD_slash_DD.test(str) ){
                matches = str.match( looksLike_MM_slash_DD_slash_DD );
                year = Number( matches[2] ) < 20 ? "20" : "19";
                str = String( matches[1] ) + year + String(matches[2]);
        }
        return str;
};
test("test autocompleteMMDDYYYYDateFormat() with bad input", function () {
        var fn = autocompleteMMDDYYYYDateFormat;
        equal(fn("bad"), "bad");
        equal(fn("123"), "123");
        equal(fn("123/1"), "123/1");
        equal(fn("1*"), "1*");
});
test("test autocompleteMMDDYYYYDateFormat() with good input", function () {
        var fn = autocompleteMMDDYYYYDateFormat;

        equal(fn("1/"), "01/");
        equal(fn("9/"), "09/");
        equal(fn("11/1/"), "11/01/");
        equal(fn("11/9/"), "11/09/");
});
test("test...