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 不加后兩個參數在FF下會報錯

 aLink.download = fileName;

 aLink.href = URL.createObjectURL(blob);

 

 aLink.dispatchEvent(evt);

 } 

downloadFile('ship.png', canvas.toDataURL("image/png"));