The Lempel-Ziv-Welch (LZW) algorithm provides loss-less data compression.

You can read a complete description of it in the   Wikipedia article   on the subject.   It was patented, but it entered the public domain in 2004.




//LZW Compression/Decompression for Strings
var LZW = {
    compress: function (uncompressed) {
        "use strict";
        // Build the dictionary.
        var i,
            dictionary = {},
            c,
            wc,
            w = "",
            result = [],
            dictSize = 256;
        for (i = 0; i < 256; i += 1) {
            dictionary[String.fromCharCode(i)] = i;
        }
 
        for (i = 0; i < uncompressed.length; i += 1) {
            c = uncompressed.charAt(i);
            wc = w + c;
            //Do not use dictionary[wc] because javascript arrays 
            //will return values for array['pop'], array['push'] etc
           // if (dictionary[wc]) {
            if (dictionary.hasOwnProperty(wc)) {
                w = wc;
            } else {
                result.push(dictionary[w]);
                // Add wc to the dictionary.
                dictionary[wc] = dictSize++;
                w = String(c);
            }
        }
 
        // Output the code for w.
        if (w !== "") {
            result.push(dictionary[w]);
        }
        return result;
    },
 
 
    decompress: function (compressed) {
        "use strict";
        // Build the dictionary.
        var i,
            dictionary = [],
            w,
            result,
            k,
            entry = "",
            dictSize = 256;
        for (i = 0; i < 256; i += 1) {
            dictionary[i] = String.fromCharCode(i);
        }
 
        w = String.fromCharCode(compressed[0]);
        result = w;
        for (i = 1; i < compressed.length; i += 1) {
            k = compressed[i];
            if (dictionary[k]) {
                entry = dictionary[k];
            } else {
                if (k === dictSize) {
                    entry = w + w.charAt(0);
                } else {
                    return null;
                }
            }
 
            result += entry;
 
            // Add w+entry[0] to the dictionary.
            dictionary[dictSize++] = w + entry.charAt(0);
 
            w = entry;
        }
        return result;
    }
}, // For Test Purposes
    comp = LZW.compress("TOBEORNOTTOBEORTOBEORNOT"),
    decomp = LZW.decompress(comp);
document.write(comp + '<br>' + decomp);