JSFiddle - React, Tailwind, and code Playground

HTML

<h3>Quick lookup for Excel style lettered headings to numerical equivalent</h3>
<label>cell reference:</label>
<input id="testes" maxlength="2" />
<label>numerical value:</label>
<input id="results" />

CSS

* {
    font-family: tahoma;
}

input {
    width: 25px;
    font-size: 16px;
}

JavaScript

$(document).ready(function() {
    $("#testes").on("keyup", function(event) {
        var str = $(this).val();
        if (str.length == 1) {
            $("#results").val(get_string_value(str));
        } else {
            var fstr     = str.slice(0,1);
            var estr     = str.slice(1,2);
            $("#results").val(get_string_value(estr) + (get_string_value(fstr) * 26));
        }
    });
});
        
function get_string_value(str) {
    return str.charCodeAt() - 96;
}