Create a base64 image from a fetch image

I would like to fetch an image and generate a base64 from it

this is what I have:

const res = await fetch('https://cdn.cnn.com/cnnnext/dam/assets/211010073527-tyson-fury-exlarge-169.jpg')
  const blob = await res.blob();
  
  console.log(blob)
  console.log(btoa(blob))

everywhere on google they say to use FileReader, but it doesn’t exist in the service workers.

Any idea how to resolve it?

thanks!