globalCompositeOperation:

your browser does not support the canvas tag

var c=document.getElementById('myCanvas');
var ctx=c.getContext('2d');
ctx.fillStyle='blue';
ctx.fillRect(10,10,50,50);
ctx.globalCompositeOperation='source-atop';
ctx.beginPath();
ctx.fillStyle='red';
ctx.arc(50,50,30,0,2*Math.PI);
ctx.fill();