SOURCE

function getBase64(imgUrl) {
  return new Promise((resolve, reject) => {
    try {
      window.URL = window.URL || window.webkitURL;
      var xhr = new XMLHttpRequest();
      xhr.open('get', imgUrl, true);
      xhr.responseType = 'blob';
      xhr.onload = function () {
        if (this.status == 200) {
          var blob = this.response;
          let oFileReader = new FileReader();
          oFileReader.onloadend = function (e) {
            let base64 = e.target.result;
            resolve(base64);
          };
          oFileReader.readAsDataURL(blob);
        }
      };
      xhr.onerror = function (err) {
        reject(err.message);
      };
      xhr.send();
    } catch (error) {
      reject(error.message);
    }
  });
}