Javascript PDFJS:pdfDoc为空

我正在尝试声明一个自定义pdf对象。以下是pdf类：

function WhiteBoardPdf(canvasContext,url){
    PDFJS.disableWorker = false;

    var pdfDoc=null,
        scale=1,
        pageNum=1;

    PDFJS.getDocument(url).then(function getPdf(_pdfDoc) {
        console.log('getting pdfDoc');
        pdfDoc = _pdfDoc;
        //self.load(_pdfDoc,scale);
        renderPage(pageNum);
    });

    function numberOfPage(){
        console.log('getting number of page');
        return pdfDoc.numPages;
    }

    function renderPage(num){
        var ctx=canvasContext;
        var canvas=ctx.canvas;
        var canvasName=canvas.className;
        var allCanvas=document.getElementsByClassName(canvasName);
        var canvasContainer=document.getElementById("whiteBoardLayerContainer");
        // Using promise to fetch the page
        pdfDoc.getPage(num).then(function(page) {
        var viewport = page.getViewport(scale);

        for(i=0;i<allCanvas.length;i++){
            allCanvas[i].height=viewport.height;
            allCanvas[i].width=viewport.width;
        }
        canvasContainer.style.width=viewport.width+'px';
        // Render PDF page into canvas context
        var renderContext = {
          canvasContext: ctx,
          viewport: viewport
        };
        page.render(renderContext);
      });
    }

    this.renderPageByPageNum=function(num){
        pageNum=num;
        renderPage(pageNum);
    }

    this.getNumberOfPages=function(){
        return numberOfPage();
    }

    this.getPdfDoc=function(){
        return pdfDoc;
    }
}

---

我无法在主程序或其他类中使用getNumberOfPages方法获取页数。有人能教我如何解决这个问题吗？

只有在调用函数getPdf_pdfDoc{后，才能调用numberOfPage。阅读有关承诺的信息/A+，网址为

getting number of page WhiteBoardPdf.js:17
Uncaught TypeError: Cannot read property 'numPages' of null WhiteBoardPdf.js:18
getting pdfDoc