Check it out here.
This is a snippet of the loop, the important part.
function loop() { updatePosition(); // Draw over the whole canvas to create the trail effect context.fillStyle = 'rgba(255, 255, 255, .05)'; context.fillRect(0, 0, canvas.width, canvas.height); // Draw the dot context.beginPath(); context.fillStyle = '#ff0000'; context.moveTo(dot.x, dot.y); context.arc(dot.x, dot.y, 3, 0, Math.PI*2, true); context.fill(); }