canvas怎么画一个正方形？

canvas画一个正方形效果如下：

<canvas id="canvas_img1">your browser does not support the canvas tag.</canvas>
<script type="text/javascript">
var canvas = document.getElementById('canvas_img1');
canvas.width = '250';
canvas.height = '100';
var ctx = canvas.getContext('2d');
ctx.strokeStyle = '#FFFF00';//空心颜色
ctx.fillStyle = '#FFFF00';//实心颜色
ctx.strokeRect(0,0,100,100);//空心
ctx.fillRect(150,0,100,100);//实心
</script>