var image = myCanvas.toDataURL("image/png").replace("image/png", "image/octet-stream"); 

image // 也可以使base64 格式的資料字串

var userAgent = navigator.userAgent; 

//判斷是否是IE11

if(userAgent.indexOf("Trident")){
var arr = image.split(',');
var mime = arr[0].match(/:(.*?);/)[1];
var bstr = atob(arr[1]);
var n = bstr.length;
var u8arr = new Uint8Array(n);
while(n--){
u8arr[n] = bstr.charCodeAt(n);
}
window.navigator.msSaveBlob(new Blob([u8arr], {type:mime}), "download.png");

}else{

//webkit

var saveLink = document.createElement('a');
saveLink.href = image;
saveLink.download = "download.png"; 
saveLink.click();
}