canvas 鼠标画线

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>cavans 鼠标画线</title>
<style>
#canvas{
    background: orange;
}
</style>
</head>
<body>
    
<canvas id="canvas" width="800px" height="400px"></canvas>

<script>

    //根据鼠标的坐标来画线
    var obj = document.getElementById('canvas');
    var oCanvas = obj.getContext('2d');
    obj.onmousedown = function(ev){
        var ev = ev || window.event;
        //选用白色笔触
        oCanvas.strokeStyle = '#fff'; 
        //起始位置
        oCanvas.moveTo(ev.clientX - obj.offsetLeft , ev.clientY - obj.offsetTop);

        document.onmousemove = function(ev){
            var ev = ev || window.event;
            //移动位置
            oCanvas.lineTo(ev.clientX - obj.offsetLeft , ev.clientY - obj.offsetTop);
            oCanvas.stroke();
        }
        document.onmouseup = function(){
            document.onmousemove = null;
            document.onmouseup = null;
        }
    }

</script>
</body>
</html>