html转图片并pdf下载（js）

html代码

<button id="renderPdf" type="button" style="height: 28px;">生成pdf</button>
<div class="" id= "mainPage" ></div>      

js代码

<script type="text/javascript" src="js/html2canvas.js"></script>
<script type="text/javascript" src="js/jsPdf.debug.js"></script>
<script type="text/javascript">
    var downPdf = document.getElementById("renderPdf");
    downPdf.onclick = function() {
        var width = $('#mainPage').width() ; //这是我们准备画的div
        var height =  $('#mainPage').height() ;
        if (teacherName){
            html2canvas(document.getElementById('mainPage'), {

                onrendered:function(canvas) {
                    var contentWidth = canvas.width - 260;
                    var contentHeight = canvas.height;

                    //一页pdf显示html页面生成的canvas高度;
                    var pageHeight = contentWidth / 595.28 * 841.89;
                    //未生成pdf的html页面高度
                    var leftHeight = contentHeight;
                    //pdf页面偏移
                    var position = 0;
                    //a4纸的尺寸[595.28,841.89]，html页面生成的canvas在pdf中图片的宽高
                    var imgWidth = 555.28;
                    var imgHeight = (555.28/contentWidth * contentHeight);

                    var pageData = canvas.toDataURL('image/jpeg', 1.0);
                    //alert(pageData);
                    console.log(pageData);
                    var pdf = new jsPDF('', 'pt', 'a4');
                    //有两个高度需要区分，一个是html页面的实际高度，和生成pdf的页面高度(841.89)
                    //当内容未超过pdf一页显示的范围，无需分页
                    if (leftHeight < pageHeight) {
                        pdf.addImage(pageData, 'JPEG', 20, 0, imgWidth, imgHeight );
                    } else {
                        while(leftHeight > 0) {
                            pdf.addImage(pageData, 'JPEG', 20, position, imgWidth, imgHeight)
                            leftHeight -= pageHeight;
                            position -= 841.89;
                            //避免添加空白页
                            if(leftHeight > 0) {
                                pdf.addPage();
                            }
                        }
                    }

                    pdf.save('result.pdf');
                }
            })
        }
        else{
            alert("必选项未填写");
        }
    }
    </script>