通过img的url转换成base64格式输出

html:

<img src="" alt="" id="demo">

js:

<script>
function toBase64(url, callback, outputFormat){//图片地址，回调，输出格式
  var canvas,ctx ;
  canvas = document.createElement('canvas');
  ctx = canvas.getContext('2d');
  img = new Image;
  img.crossOrigin = 'Anonymous';
  img.src = url;
  img.onload = function(){
    canvas.height = img.height;
    canvas.width = img.width;
    ctx.drawImage(img,0,0);
    var dataURL = canvas.toDataURL(outputFormat || 'image/jpeg');
    console.log(dataURL)
    callback.call(this, dataURL);
    canvas = null;
};
}
toBase64('http://daotechio.oss-cn-beijing.aliyuncs.com/JPG/IMG_20180907_103631_f8ecf0ac-9c82-415d-8092-a7e0c69f0457.jpg', function(base64Img){
document.getElementById('demo').setAttribute('src',base64Img)
});

</script>