JSFiddle - React, Tailwind, and code Playground

JavaScript

function vigenere(key, text) {
    var result = "";
    for (var i = 0; i < text.length; i++) {
        var k = key.charCodeAt(i % key.length);
        var c = text.charCodeAt(i);
        var offset = (c >= 97) ? 97 : 65;
        var koffset = (k >= 97) ? 97 : 65;
        if (((c - offset) >= 0) && ((c - offset) <= 26)) {
            c = (c - offset + (k - koffset) ) % 26 + offset;
        }
        result += String.fromCharCode(c);
        }
        return result;
    }

    document.body.innerHTML = (vigenere("bacon", "Meet me at the park at eleven am"))