Javascript变量的全局作用域没有按预期工作

Global Scope in Javascript Variable Not working as expected

本文关键字：工作 变量 全局 作用域 Javascript      更新时间：2023-09-26
<html>
<body>
  <div>
    <button id="prev" onclick="goPrevious()">Previous</button>
    <button id="next" onclick="goNext()">Next</button>
    &nbsp; &nbsp;
    <span>Page: <span id="page_numo"></span> / <span id="page_count"></span></span>
    <input type=text id="elNum" value=1>
  </div>
  <div>
    <canvas id="the-canvas" style="border:0px solid black"></canvas>
  </div>
  <!-- Use latest PDF.js build from Github -->
  <script type="text/javascript" src="https://raw.github.com/mozilla/pdf.js/gh-pages/build/pdf.js"></script>
  <script type="text/javascript">
    //
    // NOTE: 
    // Modifying the URL below to another server will likely *NOT* work. Because of browser
    // security restrictions, we have to use a file server with special headers
    // (CORS) - most servers don't support cross-origin browser requests.
    //
    var url = 'http://cdn.mozilla.net/pdfjs/tracemonkey.pdf';
    //
    // Disable workers to avoid yet another cross-origin issue (workers need the URL of
    // the script to be loaded, and currently do not allow cross-origin scripts)
    //
    PDFJS.disableWorker = true;
    var pdfDoc = null,
        pageNum = 1,
        scale = 0.8,
        canvas = document.getElementById('the-canvas'),
        ctx = canvas.getContext('2d');


    //
    // Get page info from document, resize canvas accordingly, and render page
    //
    function renderPage(num) {
      // Using promise to fetch the page
      pdfDoc.getPage(num).then(function(page) {
        var viewport = page.getViewport(scale);
        canvas.height = viewport.height;
        canvas.width = viewport.width;
        // Render PDF page into canvas context
        var renderContext = {
          canvasContext: ctx,
          viewport: viewport
        };
        page.render(renderContext);
      });
      // Update page counters
      document.getElementById('page_numo').textContent = pageNum;
      document.getElementById('page_count').textContent = pdfDoc.numPages;
      sic = document.getElementById('elNum');
      sic.value = pageNum;
    }
   crc = 1;
    //

    // Go to previous page
    //
    function goPrevious() {
      if (pageNum <= 1)
        return;
      pageNum--;
      crc = pageNum;      
      renderPage(pageNum);
    }
    //
    // Go to next page
    //
    function goNext() {
      if (pageNum >= pdfDoc.numPages)
        return;
      pageNum++;
      crc = pageNum;
      renderPage(pageNum);
    }
    //
    // Asynchronously download PDF as an ArrayBuffer
    //
    PDFJS.getDocument(url).then(function getPdfHelloWorld(_pdfDoc) {
      pdfDoc = _pdfDoc;
      renderPage(pageNum);
    });
   document.write ('Current page is '+crc);
    </script>
</body>
</html>

这就是代码。它是在线pdf渲染器(pdf.js)的最简单版本。我一直在研究一个项目，需要通过url将pageNum(当前pdf页码)变量传递到另一个脚本进行处理。但问题是我没有办法检索pageNum值外的renderPage函数。我认为这是Javascript变量作用域的问题，但无法看到我试图通过crc变量或通过document.getElementById('elNum').value提取有什么问题…(见文件。write ('Current page is '+crc);查看脚本末尾的第一行和第8行的字段，以查看我是如何尝试检索值的)。

谁能告诉我我做错了什么?测试页面在这里(http://jsbin.com/pdfjs-prevnext-v2/2091/edit)

注意:我看过关于这个话题的其他问题，但这似乎是一个奇怪的情况。真的需要一些帮助，希望将来有人会需要它。

document.write ('Current page is '+crc)仅在加载页面时执行。crc是一个全局变量，可以使用。

    var viewCurrPage = function() {
        console.log('Current page is '+crc);
    };
    function goPrevious() {
          if (pageNum <= 1)
            return;
          pageNum--;
          crc = pageNum;      
          renderPage(pageNum);
          viewCurrPage();
        };
function goNext() {
      if (pageNum >= pdfDoc.numPages)
        return;
      pageNum++;
      crc = pageNum;
      renderPage(pageNum);
      viewCurrPage();
    }

这与vn_grv的简单函数结合起来可能很有用

var viewCurrPage = function() {
        //console.log('Current page is '+crc);
      document.getElementById('one').innerHTML = '<a href="process.php?page='+crc+'">Process page '+crc+'</a>';
    };
//Simply adding the corresponding div like: 
<div id="one"></div>

谢谢你的回答。最好的祝福,

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