为什么canvas.toDataURL获取图片是空白

var downLoadImgUrl = $("#QRCode-item canvas")[0].toDataURL("image/png");

var _fixType = function(type) {
    type = type.toLowerCase().replace(/jpg/i, ‘jpeg‘);
    var r = type.match(/png|jpeg|bmp|gif/)[0];
    return ‘image/‘ + r;
};
var type = ‘png‘;
var fixType = _fixType(type);
downLoadImgUrl = $("#QRCode-item canvas")[0].toDataURL(fixType);
downLoadImgUrl = downLoadImgUrl.replace(fixType, ‘image/octet-stream‘);

var data = $("#QRCode-item canvas")[0].toDataURL().replace("image/png", "image/octet-stream;");//获取二维码值，并修改响应头部。
var filename="tupian.png";//保存的图片名称和格式，canvas默认使用的png格式。这个格式效果最好。
var save_link = document.createElementNS(‘http://www.w3.org/1999/xhtml‘, ‘a‘);
save_link.href = data;
save_link.download = filename;

var event = document.createEvent(‘MouseEvents‘);
event.initMouseEvent(‘click‘, true, false, window, 0, 0, 0, 0, 0, false, false, false, false, 0, null);
save_link.dispatchEvent(event);