Fetch blob and convert to base64

blob-to-base64.js

const fetchAsBlob = url => fetch(url).then(response => response.blob());

const convertBlobToBase64 = blob => new Promise((resolve, reject) => {
  const reader = new FileReader();
  reader.onerror = reject;
  reader.onload = () => {
    resolve(reader.result);
  };
  reader.readAsDataURL(blob);
});

// e.g.
fetchAsBlob('https://fonts.gstatic.com/s/roboto/v16/d-6IYplOFocCacKzxwXSOJBw1xU1rKptJj_0jans920.woff2')
 .then(convertBlobToBase64)
 .then(console.log);