http://memo.overknee.info/post/147704551073 のやつ

index.js

function consume(reader, length) {
  var total = 0;
  return (function pump() {
    console.log(`${total} / ${length}`);
    
    return reader.read().then(function(args) {
      if (!args.done) {
        total += args.value.byteLength;
        return pump();
      }
    });
  })();
};

fetch('/path/to').then(function(response) {
  var length = +response.headers.get('Content-Length');
  var reader = response.body.getReader();
  return consume(reader, length);
});