js实现canvas保存图片为png格式并下载到本地的方法

function base64Img2Blob(code){        var parts = code.split(';base64,');        var contentType = parts[0].split(':')[1];        var raw = window.atob(parts[1]);        var rawLength = raw.length;         var uInt8Array = new Uint8Array(rawLength);         for (var i = 0; i < rawLength; ++i) {         uInt8Array[i] = raw.charCodeAt(i);        }         return new Blob([uInt8Array], {type: contentType});       }      function downloadFile(fileName, content){                var aLink = document.createElement('a');        var blob = base64Img2Blob(content); //new Blob([content]);                var evt = document.createEvent("HTMLEvents");        evt.initEvent("click", false, false);//initEvent 不加后两个参数在FF下会报错        aLink.download = fileName;        aLink.href = URL.createObjectURL(blob);             aLink.dispatchEvent(evt);      }      downloadFile('ship.png', canvas.toDataURL("image/png"));