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"));