图片url转base64,该如何处理

图片url转base64
怎么把获取到的图片url转成base64
------解决思路----------------------
如果图片不跨域,可以用canvas转一下,如果跨域,网上说加上 img.crossOrigin = 'Anonymous';管用,但是我测的是不管用的

<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>canvas</title>
</head>
<body>
<img src="image/ab_photo.jpg" alt="" id="convertImg">

<script>

window.onload=function(){

function convertImgToBase64(url,fileobj){
document.createElement("div")
 var canvas=document.createElement("canvas")
 var c=canvas.getContext('2d')
img = new Image;
  img.crossOrigin = 'Anonymous';   
img.onload=function(){
  canvas.height=fileobj.height;
  canvas.width=fileobj.width;
  c.drawImage(fileobj,0,0);
  convertImg.src=canvas.toDataURL()
  
}
img.src=url;
}

var convertImg=document.getElementById("convertImg")
convertImgToBase64(convertImg.src,convertImg)
}
</script>
</body>
</html>