LZW Algorithm (JS) by Félix Cloutier (zneak)

var LZW = function(depth)
{
    var compressDict = {};
    var decompressArray = [];

    for (var i = 0; i < (1 << depth); i++)
    {
        var value = String.fromCharCode(i);
        compressDict[value] = i;
        decompressArray.push(value);
    }

    function copyCompressDict()
    {
        var dict = {};
        for (var key in compressDict) dict[key] = compressDict[key];
        return dict;
    }

    function copyDecompressArray()
    {
        var arr = [];
        for (var i = 0; i < decompressArray.length; i++)
            arr.push(decompressArray[i]);
        return arr;
    }

    this.compress = function(str)
    {
        var dict = copyCompressDict();
        var nextDictValue = 1 << depth;

        var key = '';
        var output = [];
        for (var i = 0; i < str.length; i++)
        {
            var currentChar = str[i];
            if (dict[key + currentChar] !== undefined)
            {
                key += currentChar;
            }
            else
            {
                dict[key + currentChar] = nextDictValue;
                output.push(dict[key]);
                nextDictValue++;
                key = currentChar;
            }
        }

        output.push(dict[key]);
        return output;
    }

    this.decompress = function(compressed)
    {
        var dict = copyDecompressArray();

        var key = compressed[0];
        var output = dict[key];
        var w = output;
        var entry = '';

        for (var i = 1; i < compressed.length; i++)
        {
            key = compressed[i];
            if (key < dict.length)
                entry = dict[key];
            else
                entry = w + w.charAt(0);

            output += entry;
            dict.push(w + entry.charAt(0));
            w = entry;
        }

        return output;
    }
}

var lzw = new LZW(8);
var input = "TOBEORNOTTOBEORTOBEORNOT#";
var compressed = lzw.compress(input);
console.log(input);
console.log(compressed);
console.log(lzw.decompress(compressed));