想要在html5 canvas画布中打开一张本地图片，代码有什么问题

调整循序，script放canvas后面，要不获取不到对象

   <form action="">
        <input type="file" name="imgOne" id="imgOne" onchange="preImg(this.id,'imgPre');" />
        <img id="imgPre" src="" style="display: block;" />
    </form>
    <canvas id="demo" width="400" height="400" style="border:1px solid red"></canvas>
    <script type="text/javascript">
        var canvas = document.getElementById("demo");
        var cxt = canvas.getContext("2d");
        //打开图片
        function preImg(sourceId, targetId) {
            cxt.clearRect(0, 0, 400, 400);
            if (typeof FileReader === 'undefined') {
                alert('Your browser does not support FileReader...');
                return;
            }
            var reader = new FileReader();

            reader.onload = function (e) {
                var img = document.getElementById(targetId);
                var cxt = document.getElementById("demo").getContext("2d")
                img.src = this.result;
                img.onload = function () {
                    cxt.drawImage(img, 0, 0);
                }
            }
            reader.readAsDataURL(document.getElementById(sourceId).files[0]);
        }