var img = "imgurl";//imgurl 就是你的圖片路徑
function getBase64Image(img) {
  var canvas = document.createElement("canvas");
  canvas.width = img.width;
  canvas.height = img.height;
  var ctx = canvas.getContext("2d");
  ctx.drawImage(img, 0, 0, img.width, img.height);
  var ext = img.src.substring(img.src.lastIndexOf(".")+1).toLowerCase();
          var dataURL = canvas.toDataURL("image/"+ext);
  return dataURL;
}
var image = new Image();
image.src = img;
image.onload = function(){
  var base64 = getBase64Image(image);
  console.log(base64);
}