Very slow dct (n^4) implementation on a single 8x8 macroblock. Example data based on example in Wikipedia article on JPEG

dct.js

var midpoint = 128;

var data = [
    [52, 55, 61, 66, 70, 61, 64, 73],
    [63, 59, 55, 90, 109, 85, 69, 72],
    [62, 59, 68, 113, 144, 104, 66, 73],
    [63, 58, 71, 122, 154, 106, 70, 69],
    [67, 61, 68, 104, 126, 88, 68, 70],
    [79, 65, 60, 70, 77, 68, 58, 75],
    [85, 71, 64, 59, 55, 61, 65, 83],
    [87, 79, 69, 68, 65, 76, 78, 94]
];

var shifted_data = [];
for (var x = 0; x < data.length; ++x) {
    shifted_data.push([]);
    for (var y = 0; y < data[x].length; ++y) {
        shifted_data[x].push(data[x][y] - midpoint);
    }
}

var dct = [];
for (var u = 0; u < shifted_data.length; ++u) {
    dct.push([]);
    var au = (u == 0) ? (1/Math.sqrt(2)) : 1; 
    for (var v = 0; v < shifted_data[u].length; ++v) {
        var value = 0;
        var av = (v == 0) ? (1/Math.sqrt(2)) : 1;

        for (var x = 0; x < shifted_data.length; ++x) {
            for (var y = 0; y < shifted_data[x].length; ++y) {
                value += shifted_data[x][y] * 
                    Math.cos( ((2 * x + 1) * u * Math.PI)/16 ) *
                    Math.cos( ((2 * y + 1) * v * Math.PI)/16 )
            }
        }

        value = 1/4 * av * au * value;
        dct[u].push(value);
    }
}