Simple character mapping.

charmap.js

var charMap = function charMap(str, from, to, tryLowerCase = false) {
    return str
        .split("")
        .map((char, i) => to.charAt(from.indexOf(tryLowerCase ? char.toLowerCase() : char)) || char)
        .join("");
};

var normal = "abcdefghijklmnopqrstuvwxyz",
    smallLetters = "\u1d00\u0299\u1d04\u1d05\u1d07\u0493\u0262\u029c\u026a\u1d0a\u1d0b\u029f\u1d0d\u0274\u1d0f\u1d18\u01eb\u0280s\u1d1b\u1d1c\u1d20\u1d21x\u028f\u1d22";

charMap("Lorem ipsum", normal, smallLetters);
// => "Lᴏʀᴇᴍ ɪᴘsᴜᴍ"

charMap("Lorem ipsum", normal, smallLetters, true);
// => "ʟᴏʀᴇᴍ ɪᴘsᴜᴍ"