<p id="base64-string"></p>

function imageToBase64(url, callback) {
  fetch(url)
    .then(response => response.blob())
    .then(blob => {
      const reader = new FileReader();
      reader.readAsDataURL(blob);
      reader.onloadend = () => {
        const base64String = reader.result;
        callback(base64String);
      };
    });
}
const imageUrl = 'https://picsum.photos/200';

imageToBase64(imageUrl, function(base64String){
  document.body.style.background = `url(${base64String}) no-repeat center`;
  console.log(base64String);
});