Run Length Encode & Decoder JavaScript functions - Live demo on http://www.whak.ca/demo/run-length-encode.htm

run-length-encode.js

// run length encoder/decoder, not useful when sequence has numbers already in it
function encode_rle(r) {
  for (var o, i, n = 0, t = r.length, e = ""; t > n; ++n) void 0 !== o ? r[n] === o ? i++ :(e += i + o, 
  o = r[n], i = 1) :(o = r[n], i = 1);
  return e + (i + o);
}

function decode_rle(r) {
  var t, n = /^\d+/.exec(r);
  return null === n ? "" :(n = n[0], t = r[n.length], Array(+n + 1).join(t) + decode_rle(r.substr((n + t).length)));
}

var X = "www.WHAK.com aaaaaabbbbbbbbbcbbbbbccccccddddddddaaaabbbbbaaaaacccccceeeeeeeeeee";
X = encode_rle(X);
alert(X);
X = decode_rle(X);
alert(X);

Minified version:

function r(n){var e,t=/^\d+/.exec(n);return null===t?"":(t=t[0],e=n[t.length],Array(+t+1).join(e)+r(n.substr((t+e).length)))}function n(r){for(var n,e,t=0,o=r.length,i="";o>t;++t)void 0!==n?r[t]===n?e++:(i+=e+n,n=r[t],e=1):(n=r[t],e=1);return i+(e+n)}var e="www.WHAK.com aaaaaabbbbbbbbbcbbbbbccccccddddddddaaaabbbbbaaaaacccccceeeeeeeeeee";e=n(e),e=r(e),alert(e)