test.js

    var embed = [
      'tag', 'uint32be',
      'code', 'uint32le',
      'greet', 'cstring'
    ];
    var def = [
      'tag', 'cstring:4',
      'code', 'uint32le',
      'embed', embed,
      'length', 'uint16be',
      'data', ['[]', 'float32be', function(s, ds){ return (s.length-2) / 4; }],
      'greet', 'cstring:20',
      'endNote', {
        get: function(ds, s){ return ds.readUint8(); },
        set: function(ds, v){ ds.writeUint8(v); }
      }
    ];

    var u = [137,  80,  78,  71,   0, 136, 136, 254, 137,  80,
              78,  71,   0, 136, 136, 255,  72, 101, 108, 108,
             111,  44,  32,  87, 111, 114, 108, 100,  33,   0,
               0,  10,   0,   1,   2,   3,   1,   2,   3,   4,
              72, 101, 108, 108, 111,  44,  32,  87, 111, 114,
             108, 100,  33,   0,   0,   0,   0,   0,   0,   0,
             255];

    var ds = new DataStream();
    ds.writeUint8Array(u);
    ds.seek(0);
    var obj = ds.readStruct(def);
    var p = ds.position;
    ds.writeStruct(def, obj);
    ds.seek(p);
    var obj2 = ds.readStruct(def);