原生js实现用鼠标画图

代码如下：
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>canvas鼠标画图</title>
    <style>
        body {
            background-color: yellow;
        }
        #c1{
            background-color: white;
        }
    </style>
<script>
    window.onload = function() {
        var oC = document.getElementById('c1');
        var oCG = oC.getContext('2d');
        oC.onmousedown = function(ev) {

            var ev = ev || window.event;
            oCG.moveTo(ev.clientX-oC.offsetLeft,ev.clientY-oC.offsetTop);
            //ev.clientX-oC.offsetLeft,ev.clientY-oC.offsetTop鼠标在当前画布上X,Y坐标

            document.onmousemove = function(ev) {
                var ev = ev || window.event;//获取event对象
                oCG.lineTo(ev.clientX-oC.offsetLeft,ev.clientY-oC.offsetTop);
                oCG.stroke();
            };

            oC.onmouseup = function() {
                document.onmousemove = null;
                document.onmouseup = null;
            };

        };

    };

</script>
</head>
<body>
    <canvas id="c1" width="400px" height="400px"> <!--宽高写在html里，写在css里有问题-->
        <span>该浏览器不支持canvas内容</span> <!--对于不支持canvas的浏览器显示-->
    </canvas>
</body>
</html>
效果如图：