效果显示不出来

var chess=document.getElementById('chess');

var context=chess.getContext('2d');

context.strokeStyle="#bfbfbf";

var logo=new Image();

logo.src="images/慕课图标.png";

logo.onload=function(){

context.drawImage(logo,0,0,450,450);

drawChessBoard();

context.beginPath();

context.arc(200,200,100,0,2*Math.PI);

context.closePath();

context.fill();

}

var drawChessBoard=function(){

for (var i = 0; i < 15; i++) {

context.moveTo(15+i*30,15);

context.lineTo(15+i*30,435);

context.stroke();

context.moveTo(15,15+i*30);

context.lineTo(435,15+i*30);

context.stroke();

}

}

我照代码敲的  就是图片跟棋子都出不来效果