Using Canvas with NodeJS to create a composite of images

app.js

var fs = require('fs');
var path = require('path');
var Canvas = require('canvas');
var Image = Canvas.Image;
var img = new Image();
var mask = new Image();


img.onload = function () {
  var w = img.width;
  var h = img.height;
  var canvas = new Canvas(w, h);
  var ctx = canvas.getContext('2d');

  ctx.drawImage(mask, 0, 0,w,h);
  ctx.globalCompositeOperation = "source-in";
  ctx.drawImage(img, 0, 0);

  var out = fs.createWriteStream(path.join(__dirname, 'composite.png'));
  var stream = canvas.createPNGStream({
    bufsize: 2048,
    quality: 80
  });

  stream.pipe(out);
};
mask.src = path.join(__dirname, 'composite_mask.png');
img.src = path.join(__dirname, '49ers.jpg');