不好意思，剛剛沒有看清楚你注視了代碼，重新答一次。drawstar這個函數的功能是根據指定的上下文在特定的位置繪制特定大小的的星星。根據老師前面的例子，我們最好只是規劃出一個基本圖形的單位路徑。具體的位置變換，旋轉，縮放都交給外部進行。出錯的原因是在外部你進行了旋轉，在內部進行了位移，外部的旋轉改變了里面的坐標系。將以上的代碼改為如下：

var?searchlight?=?{
	x:?400,
	y:?400,
	radius:?150,
	vx:?Math.random()?*?5?+?10,
	vy:?Math.random()?*?5?+?10
};
var?rot?=?0;
window.onload?=?function()?{
	var?canvas?=?document.getElementById('canvas');
	canvas.width?=?800;
	canvas.height?=?600;
	if?(canvas.getContext('2d'))?{
		var?context?=?canvas.getContext('2d');
	}?else?{
		alert('該瀏覽器不支持！');
	}
	setInterval(function()?{
		draw(context);
		update(canvas.width,?canvas.height);
	},?40);
};

function?draw(cxt)?{
	var?canvas?=?cxt.canvas;
	cxt.clearRect(0,?0,?canvas.width,?canvas.height);

	cxt.save();

	cxt.beginPath();
	cxt.fillStyle?=?'black';
	cxt.fillRect(0,?0,?canvas.width,?canvas.height);

	cxt.save();
	cxt.beginPath();
	//?cxt.arc(searchlight.x,searchlight.y,searchlight.radius,0,2*Math.PI);
	cxt.translate(searchlight.x,searchlight.y);
	cxt.rotate(rot/180*Math.PI);
	cxt.scale(searchlight.radius,searchlight.radius);
	drawstar(cxt);
	
	cxt.fillStyle?=?'#fff';
	cxt.fill();
	cxt.restore();

	cxt.clip();

	cxt.font?=?'bold?150px?Arial';
	cxt.textAlign?=?'center';
	cxt.textBaseline?=?'middle';
	cxt.fillStyle?=?'#058';
	cxt.fillText('CANVAS',?canvas.width?/?2,?canvas.height?/?4);
	cxt.fillText('CANVAS',?canvas.width?/?2,?canvas.height?/?2);
	cxt.fillText('CANVAS',?canvas.width?/?2,?canvas.height?/?4?*?3);
	
	cxt.restore();
}
//星星
function?drawstar(cxt)?{
	cxt.beginPath();
	for?(var?i?=?0;?i?<?5;?i++)?{
		cxt.lineTo(Math.cos((18?+?i?*?72)?/?180?*?Math.PI),?-Math.sin((18?+?i?*?72)?/?180?*?Math.PI));
		cxt.lineTo(Math.cos((54?+?i?*?72)?/?180?*?Math.PI)?*?0.5,?-Math.sin((54?+?i?*?72)?/?180?*?Math.PI)?*?0.5);
	}
	cxt.closePath();
}
//碰撞檢測
function?update(width,?height)?{
	rot?+=?1;
	searchlight.x?+=?searchlight.vx;
	searchlight.y?+=?searchlight.vy;
	if?(searchlight.x?-?searchlight.radius?<=?0)?{
		searchlight.vx?=?-searchlight.vx;
		searchlight.x?=?searchlight.radius;
	}
	if?(searchlight.x?+?searchlight.radius?>=?width)?{
		searchlight.vx?=?-searchlight.vx;
		searchlight.x?=?width?-?searchlight.radius;
	}
	if?(searchlight.y?-?searchlight.radius?<=?0)?{
		searchlight.vy?=?-searchlight.vy;
		searchlight.y?=?searchlight.radius;
	}
	if?(searchlight.y?+?searchlight.radius?>=?height)?{
		searchlight.vy?=?-searchlight.vy;
		searchlight.y?=?height?-?searchlight.radius;
	}
}

試了下你的代碼，沒有問題啊。可能是高度設置的大了點導致屏幕出現了滾動條，你會發現星星的一部分跑到了黑布的外側，你可以拖動滾動條看一下其實星星并沒有到黑布非外面，你可以試試適當設置小一點的高度，比如600就非常完美了。

會不會是瀏覽器兼容問題