<canvas id="canvas" width="100" height="100"></canvas>
<script>
var canvas = document.getElementById('canvas');
var ctx = canvas.getContext('2d');
ctx.fillStyle = 'red';
ctx.fillRect(0, 0, 100, 100);
ctx.globalCompositeOperation = 'copy';
ctx.filter = 'drop-shadow(0px 10px black)';
ctx.fillStyle = 'green';
ctx.fillRect(25, 25, 50, 40);
</script>