HTML5 사각형 그리기

function drawRect() {
     var canvas = document.getElementById("drawRCanvas");
     var context = canvas.getContext("2d");
     var left = (canvas.width - 200) * 0.5;
     var top = canvas.height * 0.1;
     var size = 200;
     context.beginPath();              
     context.fillStyle = "blue";
     context.rect(left + size * 0.2, top + size * 0.2, size * 0.6, size * 0.6);
     context.fill();                  
}