html5、canval 对 图片的压缩

let src = this.cropper.getCroppedCanvas().toDataURL('image/jpeg');
let can = document.createElement('canvas');
let img = document.createElement('img');
img.src = src;
can.width = 100;
can.height = 100;
let ctx = can.getContext("2d");
ctx.drawImage(img,0,0);
document.getElementById('previewimg1').src=can.toDataURL("image/jpeg");

function dataURLtoBlob(dataurl) {
  var arr = dataurl.split(','), mime = arr[0].match(/:(.*?);/)[1],
    bstr = atob(arr[1]), n = bstr.length, u8arr = new Uint8Array(n);
  while(n--){
    u8arr[n] = bstr.charCodeAt(n);
  }
  return new Blob([u8arr], {type:mime});
}