JSFiddle - React, Tailwind, and code Playground

HTML

var s = "Hello from Minangkabau!";
var t = new Array();
for (var i = 0; i < s.length; i++) {
    var _t; 
    switch (s[i]) {
        case 'a':
        case 'i':
        case 'u':
        case 'e':
        case 'o':
            _t = s[i].toUpperCase();
            break;
        default:
            _t = s[i].toLowerCase();
            break;
    }
    t.push(_t);
}
document.write("t is " + t);
document.write("<br />");

document.write(t.join() + "<br />");
document.write(t.join("") + "<br />");

document.write(t.toString() + "<br />");

JavaScript

var s = "Hello from Minangkabau!";
var t = new Array();
for (var i = 0; i < s.length; i++) {
    var _t; 
    switch (s[i]) {
        case 'a':
        case 'i':
        case 'u':
        case 'e':
        case 'o':
            _t = s[i].toUpperCase();
            break;
        default:
            _t = s[i].toLowerCase();
            break;
    }
    t.push(_t);
}
document.write("t is " + t);
document.write("<br />");

document.write(t.join() + "<br />");
document.write(t.join("") + "<br />");

document.write(t.toString() + "<br />");