client.js

$(function () {
  
  var canvas = document.getElementById('canvasId')
    , ctx    = canvas.getContext('2d');

   ctx.fillRect(10, 10, 10, 10);

  var imgd = ctx.getImageData(0, 0, ctx.canvas.width, ctx.canvas.height);

  var data = {
    part: 'arm',
    canvas: canvas.toDataURL()
  };

  socket.emit('canvas', data);

});

server.js

var Canvas = require('canvas')
  , Image = Canvas.Image
  , fs = require('fs');

module.exports = function (io) {

  io.sockets.on('connection', function (socket) {
    socket.on('canvas', function (data) {
    
      var canvas = new Canvas(200,200)
        , ctx = canvas.getContext('2d');

      var img  = new Image;
      
      img.onload = function () {

        ctx.drawImage(img, 0, 0, img.width, img.height);
    
        var out    = fs.createWriteStream(__dirname + '/test.png')
          , stream = canvas.createPNGStream();
    
        stream.on('data', function (chunk) {
          out.write(chunk);
        });
        
        console.log('fire?');
        
      };

      img.src = new Buffer(data.canvas.replace(/^data:image\/(png|jpg);base64,/, ""), 'base64').toString('utf8');
      // img.src = data.canvas;
  
      
    });
  });
  
};