#myCanvas
globalCompositeOperation
430
'source-over'|'source-atop'|'source-in'|'source-out'|'destination-over'|'destination-atop'|'destination-in'|'destination-out'|'lighter'|'copy'|'xor'
=
ctx.fillStyle='blue';
ctx.fillRect(10,10,50,50);
ctx.globalCompositeOperation=#val;
ctx.beginPath();
ctx.fillStyle='red';
ctx.arc(50,50,30,0,2*Math.PI);
ctx.fill();