two functions for lzw encoding and decoding.

lzwCompression.js

lzw_encode: function(s) {
      var dict = {};
	    var data = (s + '').split('');
	    var out = [];
	    var currChar;
	    var phrase = data[0];
	    var code = 256;
	    for (var i=1; i < data.length; i++) {
	        currChar=data[i];
	        if (dict[phrase + currChar] != null) {
	            phrase += currChar;
	        } else {
	            out.push(phrase.length > 1 ? dict[phrase] : phrase.charCodeAt(0));
	            dict[phrase + currChar] = code;
	            code++;
	            phrase=currChar;
	        }
	    }
	    out.push(phrase.length > 1 ? dict[phrase] : phrase.charCodeAt(0));
	    for (var i=0; i < out.length; i++) {
	        out[i] = String.fromCharCode(out[i]);
	    }
	    return out.join('');
	},

	lzw_decode: function(s) {
	    var dict = {};
	    var data = (s + '').split('');
	    var currChar = data[0];
	    var oldPhrase = currChar;
	    var out = [currChar];
	    var code = 256;
	    var phrase;
	    for (var i = 1; i < data.length; i++) {
	        var currCode = data[i].charCodeAt(0);
	        if (currCode < 256) {
	            phrase = data[i];
	        } else {
				phrase = dict[currCode] ? dict[currCode] : (oldPhrase + currChar);
	        }
	        out.push(phrase);
	        currChar = phrase.charAt(0);
	        dict[code] = oldPhrase + currChar;
	        code++;
	        oldPhrase = phrase;
	    }
	    return out.join('');
	}

function lzw_encode(r) {
  var n, h, a = {}, e = (r + "").split(""), v = [], t = e[0], o = 256;
  for (h = 1; h < e.length; h++) {
    n = e[h], null != a[t + n] ? t += n : (v.push(t.length > 1 ? a[t] : t.charCodeAt(0)), 
    a[t + n] = o, o++, t = n);
  }
  for (v.push(t.length > 1 ? a[t] : t.charCodeAt(0)), h = 0; h < v.length; h++) {
    v[h] = String.fromCharCode(v[h]);
  }
  return v.join("");
}

function lzw_decode(r) {
  var h, l, i, a = {}, e = (r + "").split(""), v = e[0], n = v, t = [ v ], o = 256;
  for (l = 1; l < e.length; l++) {
    i = e[l].charCodeAt(0), h = 256 > i ? e[l] : a[i] ? a[i] : n + v, t.push(h), v = h.charAt(0), 
    a[o] = n + v, o++, n = h;
  }
  return t.join("");
}