如何从js格式的url下载图片？

var image = new Image();

image.src = "图片的URL";

image.onload = function() {
  var canvas = document.createElement('canvas');
  canvas.width = image.width;
  canvas.height = image.height;
  var context = canvas.getContext('2d');
  context.drawImage(image, 0, 0);
};

var dataURL = canvas.toDataURL('image/png');

var a = document.createElement('a');
a.href = dataURL;

a.download = "image.png";

document.body.appendChild(a);
a.click();
document.body.removeChild(a);