getContext在谷歌浏览器中，使用时要先加载canvas对象，否则会提示'getContext is null'

<body>

<canvas id="myCanvas" width="" height="" style="border:1px solid #c3c3c3;">

Your browser does not support the canvas element.

</canvas>

    <script type="text/javascript">

        var c = document.getElementById("myCanvas");

        var cxt = c.getContext("2d");

        cxt.fillStyle = "#FF0000";

        cxt.fillRect(, , , );

    </script>

</body>

getContext在谷歌浏览器中，使用时要先加载canvas对象，否则会提示'getContext is null'