Request.blob()

request.blob().then(function(myBlob) {
  // do something with myBlob
});

const obj = {hello: 'world'};
const myBlob = new Blob([JSON.stringify(obj, null, 2)], {type : 'application/json'});

const request = new Request('/myEndpoint', {
  method: 'POST',
  body: myBlob
 });

request.blob().then(function(myBlob) {
  // do something with the blob sent in the request
});