Only encode codepoints 127 and up. 127 to 255 get encoded to hexdecimal and 256 to 65536 get encoded to unicode

encode-non-ascii.js

String.prototype.replaceWithUtf8 = function() {
  function r(r) {
    for (var t, n, e = "", i = 0; !isNaN(t = r.charCodeAt(i++)); ) n = t.toString(16), 
    e += 256 > t ? "\\x" + (t > 15 ? "" :"0") + n :"\\u" + ("0000" + n).slice(-4);
    return e;
  }
  var a, c, o, u, s, e = "", i = this, t = [ "/", '"' ], n = [ "\\/", '\\"' ];
  for (a = 0; a < i.length; a++) c = i.charCodeAt(a), o = i.charAt(a), u = t.indexOf(o), 
  u > -1 ? e += n[u] :c > 126 && 65536 > c ? (s = r(o), e += s) :e += o;
  return e;
};

USAGE:

alert("Lärm, Lichter, Lücken, Löcher.".replaceWithUtf8());