function hex(str) {

  let result = "";

  str.split("").forEach(ch => {

    result += "U+" + ("0000" + ch.codePointAt(0).toString(16).toUpperCase()).slice(-4);

  });

  return result;

}

​

function output(ch, c) {

  const chCase = c === "u" ? "Upper" : "Lower";

  const result = ch[`to${chCase}Case`]();

  console.log(`${ch} ('${hex(ch)}') → ${result} ('${hex(result)}'; to${chCase}Case)`);

}

​

output("\u00df", "l");

output("\u00df", "u");

output("\u1e9e", "l");

output("\u1e9e", "u");