未捕獲的TypeError：無法讀取null Node.js的屬性'getContext'

Question

我正在嘗試為一個項目創建流式視頻，正在包括HTML的node.js中創建，但無法通過此錯誤完成Uncaught TypeError: Can not read property 'getContext' of null

然后我需要解決該問題，但是我不能，因為我尋求解決方案和這種類型的錯誤，但是沒有一個對我有效，只是我必須在以下幾行中進行評論的問題：

  var context = canvas.getContext("2d");

        canvas.width = 800;
        canvas.height = 600;

        context.width = canvas.width;
        context.height = canvas.height

這是我的proyect的代碼：Emitir Video

    <canvas style = "display:none;" id "preview" width='1280' height='720'></canvas>

    <div id = "logger"></div>

    <script type="text/javascript">
        var canvas = document.getElementById("preview");
        var context = canvas.getContext("2d"); // This is the line that generates the error

        canvas.width = 800;
        canvas.height = 600;

        context.width = canvas.width;
        context.height = canvas.height;

        var video = document.getElementById("video");
        var socket = io();

        function logger(msg)
        {
            $("#logger").text(msg);
        }

        function loadCam(stream)
        {
            video.src = window.URL.createObjectURL(stream);
            logger('Camara cargada correctamente [OK]');
        }

        function loadFail()
        {
            logger('Camara no conectada, Favor Conectela');
        }

        function  viewVideo(video,context)
        {
            context.drawImage(video,0,0,context.width,context.height);
            socket.emit('stream',canvas.toDataURL('image/webp'));
        }

        $(function(){
            navigator.getUserMedia = (navigator.getUserMedia || navigator.webkitGetUserMedia || navigator.mozGetUserMedia || navigator.msgGetUserMedia);

            if (navigator.getUserMedia)
            {
                navigator.getUserMedia({video: true}, loadCam, loadFail);
            }
            //1000 = 1 Seg.

            setInterval(function(){
                viewVideo(video,context);
            },70);

        });
    </script>

</body>
</html>

謝謝，我將幫助您。

Answer 1

您需要將id "preview"更改為id="preview" ，然后可以繼續進行下一個錯誤：)