JSFiddle - React, Tailwind, and code Playground

JavaScript

function join(array, con) {
    con = con ? con : "";
	var str = "";
    for (var i = 0; i < array.length; i++) {
		str += array[i] + con;
    }
    return str;
}

function reverse(str) {
    var newStr = [];
    for (var i = 0; i < str.length / 2; i++) {
           var temp = str[i];
       newStr[i] = str[str.length - i - 1];
       newStr[str.length - i - 1] = temp;
    }
	return join(newStr, "");
}

document.write(reverse("FooBar"));